Fix false friends in implicit string concatenation in tests
[In a PR](https://github.com/nvim-treesitter/nvim-treesitter/pull/7788) about how to syntax-highlight Python docstrings, @rmuir and I discovered that instead of implicit concatenation of strings a string plus a docstring have been written.

Since this is a subtle one, I want to briefly show the differences:
 
The false friend of implicit string concatenation
```python
# var == "foo"

var = "foo"  # <-- no implicit string concatenation
"bar"        # <-- docstring, legal for the bytecode compiler, against PEP 257
```

can be fixed for example with surrounding brackets:
```python
# var == "foobar"

var = (
    "foo"    # <-- gets implicitly concatenated
    "bar"
)
```

I felt free to fix the ones I found right away such that the tests pass.

I searched with [`ripgrep`](https://github.com/BurntSushi/ripgrep) like so:

```sh
# in path/to/cloned/pandas
rg -A 1 -B 2 -U ' = f?"[^"]*"\s+f?"[^"]+"\s*' pandas/
```

I am pretty confident, but not entirely sure, to have catched all cases. :thinking: 
As a side note:

The tests passed before because they called [`assert_produces_warning`](https://github.com/pandas-dev/pandas/blob/main/pandas/_testing/_warnings.py#L29), which in turn calls [`_assert_caught_expected_warnings`](https://github.com/pandas-dev/pandas/blob/main/pandas/_testing/_warnings.py#L157), where `re.search` is used:

https://github.com/pandas-dev/pandas/blob/04356be7d385dcb99d3040e37a85e1030afb259b/pandas/_testing/_warnings.py#L181-L183

The false friends only gave the first part of the full string to match (the rest was discarded as it was detected as docstring), but `re.search` still gives a positive match in that case.

Maybe it should be `re.fullmatch` instead? :shrug:  > I am pretty confident, but not entirely sure, to have catched all cases. 🤔

I rechecked the logfile from the script I ran yesterday: these are the same files and lines with highlight differences, so I think you found them all. Thanks @jbdyn 