ldbl-128ibm-compat: Add tests for strfroml, strtold, and wcstold

Since the commit

commit 86a0f561
Author: Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>
Date:   Thu Jun 28 13:57:50 2018 +0530

    ldbl-128ibm-compat: Introduce ieee128 symbols

IEEE long double versions of strfroml, strtold, and wcstold have been
prepared, but not exposed (which will only happen when the full support
for IEEE long double is complete).  This patch adds tests for these
functions in both IBM and IEEE long double mode.

Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>