libstdc++: Rename tests with incorrect extension

The libstdc++ testsuite only runs .cc files, so these two old tests have
never been run.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/valarray/dr630-3.C: Moved to...
	* testsuite/26_numerics/valarray/dr630-3.cc: ...here.
	* testsuite/27_io/basic_iostream/cons/16251.C: Moved to...
	* testsuite/27_io/basic_iostream/cons/16251.cc: ...here.

(cherry picked from commit 749c31b345c2a37106b57ce805ea46a6d4765e09)

diff --git a/libstdc++-v3/testsuite/26_numerics/valarray/dr630-3.C b/libstdc++-v3/testsuite/26_numerics/valarray/dr630-3.cc
similarity index 100%
rename from libstdc++-v3/testsuite/26_numerics/valarray/dr630-3.C
rename to libstdc++-v3/testsuite/26_numerics/valarray/dr630-3.cc

diff --git a/libstdc++-v3/testsuite/27_io/basic_iostream/cons/16251.C b/libstdc++-v3/testsuite/27_io/basic_iostream/cons/16251.cc
similarity index 100%
rename from libstdc++-v3/testsuite/27_io/basic_iostream/cons/16251.C
rename to libstdc++-v3/testsuite/27_io/basic_iostream/cons/16251.cc