MDEV-11619: mtr --mem {no arguement of a directory}

[grooverdan]

mtr --mem plugin was an issue because plugin was treated
as a directory where mtr treated as a tmp location rather
than a legitimate test name.

This removes functionality of where ./mtr --mem /tmp/dir could be
a directory.

Now MTR_MEM=/tmp/dir ./mtr is needed.

The case where MTR_MEM=/tmp/dir ./mtr --mem has the equalivant
effect.

Thanks Elena for helping and identifing edge cases.

I submit this under the MCA.

[elenst]

Thanks. There is also this fragment to remove:

+    # however if the opt_mem was given a value, use this first
+    unshift(@tmpfs_locations, $opt_mem) if $opt_mem ne '';