Update fs_mgr tests for new fstab format and add to TEST_MAPPING

And fix a bug in the meantime, where mounts with no filesystem
specific mount options were incorrectly having an empty string in
their set of mount options.

Test: this test
Change-Id: I9b1f14d00a90f8b95a13fcecb3cfa7fe10a2f96a
diff --git a/TEST_MAPPING b/TEST_MAPPING
index f9c3b4a..bc5685b 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -7,6 +7,9 @@
       "name": "debuggerd_test"
     },
     {
+      "name": "fs_mgr_unit_test"
+    },
+    {
       "name": "init_tests"
     },
     {