TST/CI: Unify CI test scripts (single and multi) and simplify the system
- [X] refs #23658
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

The goal of this PR is:
- Avoid repetitive code by merging `ci/script_single.sh` and `ci/script_multi.sh`
- Simplify calls to different tests (mainly slow vs not slow), by avoiding the previously two different syntax (`SLOW=true` and `--only-slow`) and using the pytest native syntax instead
- Integrate `upload_coverage.sh` into the new unified script to avoid the extra complexity of having many scripts

Hello @datapythonista! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/conftest.py`](https://github.com/datapythonista/pandas/blob/12bb05948215b10e2fa1d573de2888f9d2256b46/pandas/conftest.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23924?src=pr&el=h1) Report
> Merging [#23924](https://codecov.io/gh/pandas-dev/pandas/pull/23924?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6c3d9cbc906fb3d21cbeaf10d58996b6b600f2af?src=pr&el=desc) will **decrease** coverage by `49.84%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23924/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23924?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #23924       +/-   ##
===========================================
- Coverage    92.3%   42.45%   -49.85%     
===========================================
  Files         161      161               
  Lines       51551    51549        -2     
===========================================
- Hits        47585    21886    -25699     
- Misses       3966    29663    +25697
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `42.45% <ø> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23924?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-97.64%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-95.46%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.17%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `0% <0%> (-90.15%)` | :arrow_down: |
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `10.44% <0%> (-89.56%)` | :arrow_down: |
| ... and [119 more](https://codecov.io/gh/pandas-dev/pandas/pull/23924/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23924?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23924?src=pr&el=footer). Last update [6c3d9cb...8c9b557](https://codecov.io/gh/pandas-dev/pandas/pull/23924?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback looks like the doc build failed: https://travis-ci.org/pandas-dev/pandas/builds/461388989?utm_source=github_status&utm_medium=notification

i restarted, let's see. I didn't see the docs failed. I forgot to skip the tests in the doc build, and it was timing out doing both things.

It should be fixed now. thanks @datapythonista 