libstdc++: Move std::bitset test to correct directory

I added this test in r16-3435-gbbc0e70b610f19 but I'd previously moved
the rest of the bitset tests under 20_util, in r13-2778-g4b4b51445f7f3d.
This moves the lwg4294.cc test where it belongs.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/bitset/lwg4294.cc: Move to...
	* testsuite/20_util/bitset/cons/lwg4294.cc: ...here.

diff --git a/libstdc++-v3/testsuite/23_containers/bitset/lwg4294.cc b/libstdc++-v3/testsuite/20_util/bitset/cons/lwg4294.cc
similarity index 100%
rename from libstdc++-v3/testsuite/23_containers/bitset/lwg4294.cc
rename to libstdc++-v3/testsuite/20_util/bitset/cons/lwg4294.cc