TST: cover search_sorted scalar mixed timezones case

[jamescobonkerr]

• closes Series.searchsorted with different timezones #30086
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Raises a ValueError on version 0.25.1 (reporter's version), as expected:

>>> import pandas as pd
>>> pd.__version__
'0.25.1'
>>> ser = pd.Series(date_range("20120101", periods=10, freq="2D", tz="UTC"))
>>> val = pd.Timestamp("20120102", tz="America/New_York")
>>> res = ser.searchsorted(val)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/jamescobonkerr/venv/lib/python3.7/site-packages/pandas/core/series.py", line 2694, in searchsorted
    return algorithms.searchsorted(self._values, value, side=side, sorter=sorter)
  File "/Users/jamescobonkerr/venv/lib/python3.7/site-packages/pandas/core/algorithms.py", line 1887, in searchsorted
    result = arr.searchsorted(value, side=side, sorter=sorter)
  File "/Users/jamescobonkerr/venv/lib/python3.7/site-packages/pandas/core/arrays/datetimelike.py", line 666, in searchsorted
    self._check_compatible_with(value)
  File "/Users/jamescobonkerr/venv/lib/python3.7/site-packages/pandas/core/arrays/datetimes.py", line 591, in _check_compatible_with
    own=self.tz, other=other.tz
ValueError: Timezones don't match. 'UTC != America/New_York'

[jreback]

thanks @jamescobonkerr