test: fix fd_is_mount_point() check

So the currentl and only fd_is_mount_point() check is actually entirely
bogus: it passes "/" as filename argument, but that's not actually a
a valid filename, but an absolute path.

fd_is_mount_point() is written in a way tha the fd refers to a directory
and the specified path is a file directly below it that shall be
checked. The test call actually violated that rule, but still expected
success.

Let's fix this, and check for this explicitly, and refuse it.

Let's extend the test and move it to test-mountpoint-util.c where the
rest of the tests for related calls are placed.

Replaces: #18004
Fixes: #17950
(cherry picked from commit 95231c72)