tests: fix compiler warnings on Windows with mingw

tests: fix compiler warnings on Windows with mingw

With commit 3f184c3abc55, the gst_dir variable becomes unusable in
windows build. Moving it to linux scope to avoid warning:

    [433/673] Compiling C object 'tests/check/7d01337@@libs_profile@exe/libs_profile.c.obj'.
    ../tests/check/libs/profile.c: In function 'profile_suite':
    ../tests/check/libs/profile.c:688:10: warning: unused variable 'gst_dir' [-Wunused-variable]
       gchar *gst_dir;
             ^~~~~~~

Also fix a typo in the comment.