This test is intended to verify the fix for
https://bugzilla.gnome.org/show_bug.cgi?id=787731, which was that
g_file_query_filesystem_info() would return stale information for the
mount. After replacing a read-only mount with a read-write mount, this
test used to only fail if G_FILE_ATTRIBUTE_FILESYSTEM_READONLY was TRUE
and yet the file could be opened for writing. In particular, if (due to
a test bug) the file really was still on a read-only filesystem, the
test would pass.

Now that we have fixed that bug in the test, we can make a stronger
assertion.