[lldb][windows] Make skipUnlessMSVC tolerate cl.exe not in PATH (#198290)
If `cl.exe` is not in the PATH, `subprocess.run([\"cl.exe\"])` raises
`FileNotFoundError`. This marks the test as `UNRESOLVED` instead of
`SKIPPED`.
This patch makes sure lit catches `FileNotFoundError` so the test is
skipped cleanly.