tests: search mount point in canonicalized form in /proc/mounts

tests: search mount point in canonicalized form in /proc/mounts

* tests/functions.sh (ts_is_mounted): Enhance to resolve symlinks in
given mount point before grep'ing in /proc/mount.

If the test directory is on a symlink, then e.g. tests/ts/cramfs/doubles
failed because the kernel keeps the mount entry in a canonicalized form
in /proc/mounts while this function searched for it with the original
path name.

Signed-off-by: Bernhard Voelker <mail@bernhard-voelker.de>