tests/run-tests.py: Implement getcwd on __FS hook filesystem.

This method is needed by tests like `extmod/vfs_rom.py`.

Signed-off-by: Damien George <damien@micropython.org>

tests/run-tests.py

@@ -84,6 +84,8 @@ class __FS:
     pass
   def chdir(self, path):
     pass
+  def getcwd(self):
+    return ""
   def stat(self, path):
     if path == '__injected_test.mpy':
       return tuple(0 for _ in range(10))