libstdc++: Make some #error strings consistent with other tests

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/lerp.cc: Add header name to #error.
	* testsuite/26_numerics/midpoint/integral.cc: Likewise.
	* testsuite/26_numerics/midpoint/version.cc: New test.