libstdc++: Move stacktrace tests to 19_diagnostics directory

This matches where the feature is defined in the current draft.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/stacktrace/entry.cc: Moved to...
	* testsuite/19_diagnostics/stacktrace/entry.cc: ...here.
	* testsuite/20_util/stacktrace/synopsis.cc: Moved to...
	* testsuite/19_diagnostics/stacktrace/synopsis.cc: ...here.
	* testsuite/20_util/stacktrace/version.cc: Moved to...
	* testsuite/19_diagnostics/stacktrace/version.cc: ...here.

diff --git a/libstdc++-v3/testsuite/20_util/stacktrace/entry.cc b/libstdc++-v3/testsuite/19_diagnostics/stacktrace/entry.cc
similarity index 100%
rename from libstdc++-v3/testsuite/20_util/stacktrace/entry.cc
rename to libstdc++-v3/testsuite/19_diagnostics/stacktrace/entry.cc

diff --git a/libstdc++-v3/testsuite/20_util/stacktrace/synopsis.cc b/libstdc++-v3/testsuite/19_diagnostics/stacktrace/synopsis.cc
similarity index 100%
rename from libstdc++-v3/testsuite/20_util/stacktrace/synopsis.cc
rename to libstdc++-v3/testsuite/19_diagnostics/stacktrace/synopsis.cc

diff --git a/libstdc++-v3/testsuite/20_util/stacktrace/version.cc b/libstdc++-v3/testsuite/19_diagnostics/stacktrace/version.cc
similarity index 100%
rename from libstdc++-v3/testsuite/20_util/stacktrace/version.cc
rename to libstdc++-v3/testsuite/19_diagnostics/stacktrace/version.cc