testsuite/plugins: Add multi_cpu_race modifier on Windows

A few tests previously failed with various failure modes. For instance,
  `plugin-recomp-change` fails with:

```
Wrong exit code for plugin-recomp-change()(expected 0 , actual 2 )
Stderr ( plugin-recomp-change ):
Simple Plugin Passes Queried
Got options:
Simple Plugin Pass Run
C://GitLabRunner//builds//8fc0e283//0//ghc//ghc//inplace//mingw//bin/ld.exe: cannot find -lHSplugin-recompilation-0.1-CPeObcGoBuvHdwBnpK9jQq
collect2.exe: error: ld returned 1 exit status
`gcc.exe' failed in phase `Linker'. (Exit code: 1)
make[2]: *** [Makefile:112: plugin-recomp-change] Error 1
*** unexpected failure for plugin-recomp-change(normal)
```

It's unclear whether the ghc-pkg concurrency issue mentioned in all.T is
the culprit but the set of tests that fail overlaps strongly with the
set of tests that lack the `multi_cpu_race` modifier. Let's see if
adding it fixes them.