LD/testsuite: Fix archive verification for Alpha/VMS

LD/testsuite: Fix archive verification for Alpha/VMS

As from commit a2298f23f2e2 ("PR 33385 vms archives") the handling of
Alpha/VMS archives has been recently unbroken, after 7 years, revealing
issues with archive tests added with commit 435223a9ae70 ("LD/testsuite:
Add tests for archive handling"), the change of the failure mode of
which went unnoticed.

In particular there is no NM support for executables and archives always
have an index by the nature of the format; cf. `_bfd_vms_lib_archive_p':
"The map is always present."

Address these peculiarities by excluding NM tests for the executables
produced and by omitting mapless archive rejection tests just as with
XCOFF targets, removing all the test failures:

alpha-dec-vms  -FAIL: Regular archive link
alpha-dec-vms  -FAIL: Thin archive link
alpha-dec-vms  -FAIL: Regular archive plus regular link
alpha-dec-vms  -FAIL: Regular archive plus thin link
alpha-dec-vms  -FAIL: Thin archive plus regular link
alpha-dec-vms  -FAIL: Thin archive plus thin link
alpha-dec-vms  -FAIL: Regular archive w/o index link
alpha-dec-vms  -FAIL: Thin archive w/o index link

Factor out code to filter out unwanted tests to a procedure for a better
structure of the code.