Avoid double quotes in switchM.t runperl args.

The new tests added in 9fdd5a7ac74817 were sending runperl a -e
command with double quotes around it, which confuses matters on
VMS where arguments get double quotes added unless they already
have them.  The solution is simple: just use the 'prog' parameter
to runperl and let runperl handle the quoting.
Make the test suite pass with -DPERL_DISABLE_PMC

Commit 9fdd5a7ac74817cfaab6 introduced new tests that fail when building
perl without PMC support.  In such cases, skip these new tests.