Fix regression test handling of test_decoding with MSVC

Fix regression test handling of test_decoding with MSVC

The set of scripts in charge of running the regression tests for MSVC
run currently under the assumption that only $(top_builddir) can used in
option values defined in REGRESS_OPTS, and those options need to have a
specific format as well to be correctly parsed, so fix the Makefile
values so as those are correctly set.

Per complains from buildfarm member dory and whelk, with some extra
testing done on my side with MSVC to check this patch.