tests: Skip failing tests on MSVC

tests: Skip failing tests on MSVC

Test 'deplibs without file command' fails on MSVC with a 'hard failure'.
Since this test was created for MinGW, skipping for now.

Test 'Libtool stress test.' fails on MSVC when the cl compiler cannot
open an expsym file, which appears to exist but is empty.

* tests/deplibs-mingw.at: Add check for MSVC build to skip test.
* tests/stresstest.at: Add check for MSVC build to skip test.