testsuite: Enrich tests with variants failing on the branch.

Backporting the fix for pr103789 on the 11 branch revealed a lack of test
coverage for the tests provided with that fix.  Indeed, the tests use the KIND
argument of the respective intrinsics only with keyword arguments.
This adds variants with non-keyword arguments.

The tests enriched this way fail on the branch if the fix is cherry-picked
straightforwardly.  The fix will have to be tweaked slightly there.

	PR fortran/103789
	PR fortran/87711
	PR fortran/97896

gcc/testsuite/ChangeLog:

	* gfortran.dg/index_5.f90: Enrich test with usages of INDEX with
	a non-keyword KIND argument.
	* gfortran.dg/len_trim.f90: Same for LEN_TRIM.
	* gfortran.dg/maskl_1.f90: Same for MASKL.
	* gfortran.dg/maskr_1.f90: Same for MASKR.
	* gfortran.dg/scan_3.f90: Same for SCAN.
	* gfortran.dg/verify_3.f90: Same for VERIFY.