tests/extmod: Add test for importing a script from a user VFS.

tests/extmod/vfs_fat_more.py

@@ -114,3 +114,11 @@ uos.umount('/')
 print(uos.getcwd())
 print(uos.listdir())
 print(uos.listdir('sys'))
+
+# test importing a file from a mounted FS
+import sys
+sys.path.clear()
+sys.path.append('/sys')
+with open('sys/test_module.py', 'w') as f:
+    f.write('print("test_module!")')
+import test_module

tests/extmod/vfs_fat_more.py.exp

@@ -26,3 +26,4 @@ mkdir OSError True
 /
 ['sys']
 []
+test_module!