The problem was that main.information_schema_cs (and a handful of other tests)
could sometimes fail due to missing characterset comments.

The root cause was the regression test for Bug#16204175 in main.ctype_uca.
This test temporarily replaced share/charset/Index.xml in the source directory
with its own file. If this happened to coincide with other tests depending
on the original contents of Index.xml, those tests would fail.

This patch fixes the problem by rewriting the regression test for Bug#16204175
such that it leaves Index.xml intact and instead restarts the server with
--character-sets-dir pointing to a different directory. This is similar to
what is already done in e.g. main.invalid_collation.

See also duplicate

Bug#19372763 SPORADIC FAILURE IN MAIN.INTERNAL_TMP_DISK_STORAGE_ENGINE

Backport patch from 8.0.

Change-Id: Ic1eef7b8f448ecf4620d77123e247f106fcd328a