libgomp testsuite: Use 'lang_test_file_found' instead of 'lang_test_file'

The value of 'lang_test_file' isn't actually used anywhere.

	libgomp/
	* testsuite/libgomp.c++/c++.exp: Don't set 'lang_test_file'.
	* testsuite/libgomp.fortran/fortran.exp: Likewise.
	* testsuite/libgomp.oacc-c++/c++.exp: Likewise.
	* testsuite/libgomp.oacc-fortran/fortran.exp: Likewise.
	* testsuite/libgomp.c/c.exp: Unset 'lang_test_file_found' instead of
	'lang_test_file'.
	* testsuite/libgomp.oacc-c/c.exp: Likewise.
	* testsuite/libgomp.graphite/graphite.exp: Likewise.
	* testsuite/lib/libgomp.exp (libgomp_target_compile): Look for
	'lang_test_file_found' instead of 'lang_test_file'.