TST: add cases for nearest_workday and before_nearest_workday

[ivanovmg]

• xref TST: Add tests for observance rules in holiday.py #38139
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Add cases for testing observance rules nearest_workday and before_nearest_workday for their full coverage.

[jreback]

can you merge master, CI looks to be acting up.

[ivanovmg]

Linux tests are red due to a failure to build environment (new pip).
Windows test failure is irrelevant.

    @tm.network
    @pytest.mark.single
    def test_round_trip_exception_(self):
        # GH 3867
        csv = "https://raw.github.com/hayd/lahman2012/master/csvs/Teams.csv"
>       df = pd.read_csv(csv)

[simonjayhawkins]

Linux tests are red due to a failure to build environment (new pip).
Windows test failure is irrelevant.

re-run and green

[jreback]

thanks @ivanovmg