CI: transition powershell-> bash
xref #26344

If this works, i think the remaining places to bashify are
- identical powerhsell commands in windows.yml and posix.yml to check for test failures
- conda.recipe/bld.bat (do we need this directory at all?)
- test.bat, test_fast.bat (do we need the bash versions of these?)
Thanks @jbrockmendel  @jreback any idea if we need conda.recipe or test/test_fast files mentioned in the OP?  (or binstar.yml, that looks you-specific) binstar is really old can remove

the test_fast are local scripts
need a canonical way to run all tests
how do u usually do it?  how do u usually do it?

In my .bash_aliases file i have

```
gitd () {
	BRANCH=$1
	rm -rf ../$BRANCH
	git worktree prune
	git branch -D $BRANCH
}

pbranch () {
	BRANCH=$1
	git branch $BRANCH
	git worktree add ../$BRANCH $BRANCH
	cd ..
	cd $BRANCH
	python3 setup.py build_ext --inplace
}

alias ptest="python3 -m pytest pandas/tests --skip-slow --skip-db"
alias pflake="git diff --name-only | grep '\.py$' | xargs flake8"
alias psort="git diff --name-only | grep '\.py$' | xargs isort"
alias pblack="git diff --name-only | grep '\.py$' | xargs black"
alias pbuild="python3 setup.py build_ext --inplace"
alias ppull="git pull upstream master"

``` >  need a canonical way to run all tests

if we want to support this robustly, we might want to consider going back to `tox` @jbrockmendel the one disadvantage to your commands versus pytest would be the lack of parallel execution (i.e. `-n 8` flag). Indifferent on getting rid of the file but if we do probably worth specifying the parallel execution ability as it can save significant time when running tests so would be nice to push some of these commands to the makefile:)

also not averse to having. single canonical test_fast.sh (which we basically have now)