libstdc++: Replace _CharT template parameter with CharT in format tests.

libstdc++: Replace _CharT template parameter with CharT in format tests.

As pointed out by Daniel Krügler we do not need to use reserved name
in tests.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/vector/bool/format.cc: Replaced _CharT
	with CharT.
	* testsuite/std/format/debug.cc: Likewise.
	* testsuite/std/format/ranges/adaptors.cc: Likewise.
	* testsuite/std/format/ranges/formatter.cc: Likewise.
	* testsuite/std/format/ranges/map.cc: Likewise.
	* testsuite/std/format/ranges/sequence.cc: Likewise.
	* testsuite/std/format/ranges/string.cc: Likewise.
	* testsuite/std/format/tuple.cc: Likewise.
	* testsuite/std/time/format/empty_spec.cc: Likewise.
	* testsuite/std/time/format/pr120114.cc: Likewise.
	* testsuite/std/time/format/pr120481.cc: Likewise.
	* testsuite/std/time/format/precision.cc: Likewise.