tests: Replace $TEST_FUNCTIONS

Replace strange $TEST_FUNCTIONS variable/expansion thing with
something more like what we use in nbdkit, a simple tests/functions.sh
script that gets sourced into each test script.

Update the common submodule to get:

  commit 8137d47d0e654065391151eb275e3b64f230f6f5
  Author: Richard W.M. Jones <rjones@redhat.com>
  Date:   Thu Feb 13 11:13:55 2025 +0000

    mlcustomize, mltools: Replace $TEST_FUNCTIONS

    TEST_FUNCTIONS is being removed from libguestfs and guestfs-tools (it
    was removed from virt-v2v a while back).  Make the same adjustment in
    the common submodule.

(and some other commits which are not relevant to libguestfs)

fuse/test-docs.sh

@@ -18,7 +18,10 @@
 
 set -e
 
-$TEST_FUNCTIONS
+source ../tests/functions.sh
+set -e
+set -x
+
 skip_if_skipped
 
 $top_srcdir/podcheck.pl "$srcdir/guestmount.pod" guestmount \

fuse/test-fuse-umount-race.sh

@@ -19,9 +19,10 @@
 # https://bugzilla.redhat.com/show_bug.cgi?id=838592
 # This tests that the --pid-file option can be used to fix the race.
 
+source ../tests/functions.sh
 set -e
+set -x
 
-$TEST_FUNCTIONS
 skip_if_skipped "test-fuse.sh"
 skip_if_skipped
 skip_unless_phony_guest fedora.img

fuse/test-guestunmount-not-mounted.sh

@@ -20,7 +20,10 @@
 # Test that guestunmount returns the correct error code if
 # there is no mounted FUSE filesystem.
 
-$TEST_FUNCTIONS
+source ../tests/functions.sh
+#set -e  # command below deliberately fails
+set -x
+
 skip_if_skipped "test-fuse.sh"
 skip_if_skipped
 skip_unless_fuse