fs: check for symlink support in fs-promises test

Attempting to make symlinks as a non-administrator user in Windows causes a permission error. We need to update test-fs-promises to expect this error case, as is already done in various other tests.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

• fs