Update of --mem flag after comments

mysql-test/mysql-test-run.pl:
  Allow --mem to be specified as a flag or --mem=<dir>
  Add /tmp to locations to search
  Update comments and usage description

mysql-test/mysql-test-run.pl

@@ -667,7 +667,7 @@ sub command_line_setup () {
              'tmpdir=s'                 => \$opt_tmpdir,
              'vardir=s'                 => \$opt_vardir,
              'benchdir=s'               => \$glob_mysql_bench_dir,
-             'mem'                      => \$opt_mem,
+             'mem:s'                    => \$opt_mem,
 
              # Misc
              'comment=s'                => \$opt_comment,
@@ -747,17 +747,18 @@ sub command_line_setup () {
   # --------------------------------------------------------------------------
   # Check if we should speed up tests by trying to run on tmpfs
   # --------------------------------------------------------------------------
-  if ( $opt_mem )
+  if ( defined $opt_mem )
   {
     mtr_error("Can't use --mem and --vardir at the same time ")
       if $opt_vardir;
     mtr_error("Can't use --mem and --tmpdir at the same time ")
       if $opt_tmpdir;
 
-    # Use /dev/shm as the preferred location for vardir and
-    # thus implicitly also tmpdir. Add other locations to list
-    my @tmpfs_locations= ($opt_mem, "/dev/shm");
-    # One could maybe use "mount" to find tmpfs location(s)
+    # Search through list of locations that are known
+    # to be "fast disks" to list to find a suitable location
+    # Use --mem=<dir> as first location to look.
+    my @tmpfs_locations= ($opt_mem, "/dev/shm", "/tmp");
+
     foreach my $fs (@tmpfs_locations)
     {
       if ( -d $fs )
@@ -4630,9 +4631,12 @@ Options to control directories to use
   vardir=DIR            The directory where files generated from the test run
                         is stored (default: ./var). Specifying a ramdisk or
                         tmpfs will speed up tests.
-  mem                   Run testsuite in "memory" using tmpfs if
-                        available(default: /dev/shm)
-                        reads path from MTR_MEM environment variable
+  mem[=DIR]             Run testsuite in "memory" using tmpfs or ramdisk
+                        Attempts to use DIR first if specified else
+                        uses as builtin list of standard locations
+                        for tmpfs (/dev/shm)
+                        The option can also be set using environment
+                        variable MTR_MEM=[DIR]
 
 Options to control what test suites or cases to run