tests: unskip a test on msys2 that now works

See mesonbuild#9532 (comment)
for context.

It's unclear why this was skipped exactly in the first place, but it started to
pass recently and MSYS2 fixed some cmake+llvm related things in
msys2/MINGW-packages@345fe43
at the same time, so this is likely the reason.

test cases/frameworks/15 llvm/test.json

@@ -3,7 +3,7 @@
     "options": {
       "method": [
         { "val": "config-tool", "skip_on_jobname": ["msys2-gcc"]},
-        { "val": "cmake", "skip_on_jobname": ["msys2"] }
+        { "val": "cmake" }
       ],
       "link-static": [
         { "val": true, "skip_on_jobname": ["opensuse"] },