testing: add Chdir

Some tests need to use os.Chdir, but the use is complicated because
 - they must change back to the old working directory;
 - they must not use t.Parallel.

Add Chdir that covers these cases, and sets PWD environment variable
to the new directory for the duration of the test for Unix platforms.
Unify the panic message when t.Parallel is used together with t.Setenv
or t.Chdir.

Add some tests.

For #62516.

Change-Id: Ib050d173b26eb28a27dba5a206b2d0d877d761c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/529895


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Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>