tests: make new -mount test more robust

This test failed on non-Linux systems:
- On Solaris 11, the output of the native df(1) tool has a different order,
  and hence the detection of a usable mount point for the test fails.
  Verify that df(1) is from GNU coreutils, or fall back to 'gdf',
  else skip the test.
- On a FreeBSD system where /home was a symlink to /usr/home, the code
  for finding a usable mount point failed, because the symlink itself
  is on the '/' file system.
  Ensure that the found mount point is identical to the original test
  directory like /home etc.

* tests/find/mount-vs-xdev.sh: Try harder to use a GNU df(1) tool.
Check whether the found mount point is identical to the original
directory name, thus avoiding symlinks.

tests/find/mount-vs-xdev.sh

@@ -19,9 +19,17 @@
 . "${srcdir=.}/tests/init.sh"; fu_path_prepend_
 print_ver_ find
 
+# Require GNU df in getmntpoint.
+for f in df gdf; do
+  # Find GNU df.
+  $f --version | grep GNU \
+    && DF=$f
+done
+test "$DF" || skip_ "GNU df required."
+
 getmntpoint () {
   # Skip header line and print last field.
-  df "$1" | awk 'NR==2 {print $NF}'
+  $DF "$1" | awk 'NR==2 {print $NF}'
 }
 
 mnt_root=$( getmntpoint '/' ) \
@@ -32,9 +40,10 @@ found=0
 # Find a directory entry which is mounted (likely) from a different device
 # than the '/' directory.
 for m in /dev /home /proc /run /tmp; do
-  test -e "$m" \
+  test -d "$m" \
     && mnt_m=$( getmntpoint "$m" )  \
     && test "$mnt_root" != "$mnt_m" \
+    && test "$m" = "$mnt_m" \
     || continue
 
   found=1