MDEV-11619: mtr --mem {no argument of a directory} (#320)

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 equivalent effect.
Signed-off-by: Daniel Black <daniel.black@au.ibm.com>

--mem works better as a pure flag, because it can be followed by command-line arguments (test names). If the option is allowed to have a value, the test name which directly follows it will be treated as the option value instead. It is possible to implement workarounds to avoid this, but they would not be completely reliable, and there is no practical purpose of such extension of functionality to justify them.

mysql-test/mysql-test-run.pl

@@ -1194,7 +1194,7 @@ sub command_line_setup {
 	     # Directories
              'tmpdir=s'                 => \$opt_tmpdir,
              'vardir=s'                 => \$opt_vardir,
-             'mem:s'                    => \$opt_mem,
+             'mem'                      => \$opt_mem,
 	     'clean-vardir'             => \$opt_clean_vardir,
              'client-bindir=s'          => \$path_client_bindir,
              'client-libdir=s'          => \$path_client_libdir,
@@ -1457,9 +1457,6 @@ sub command_line_setup {
     # Use $ENV{'MTR_MEM'} as first location to look (if defined)
     unshift(@tmpfs_locations, $ENV{'MTR_MEM'}) if defined $ENV{'MTR_MEM'};
 
-    # however if the opt_mem was given a value, use this first
-    unshift(@tmpfs_locations, $opt_mem) if $opt_mem ne '';
-
     foreach my $fs (@tmpfs_locations)
     {
       if ( -d $fs && ! -l $fs )