Bug #59489 Enable setting of env. variables for mysqld from mtr

Added --mysqld-env option, propagate via safe_process
Simplified: should be safe to set in parent safe_process after it's started
Addendum: catch cases of --mysqld-env w/o value, assume env.var 
    name never begins with "--"

mysql-test/mysql-test-run.pl

@@ -176,6 +176,7 @@ our $opt_big_test= 0;
 our @opt_combinations;
 
 our @opt_extra_mysqld_opt;
+our @opt_mysqld_envs;
 
 my $opt_compress;
 my $opt_ssl;
@@ -962,6 +963,7 @@ sub command_line_setup {
 
              # Extra options used when starting mysqld
              'mysqld=s'                 => \@opt_extra_mysqld_opt,
+             'mysqld-env=s'             => \@opt_mysqld_envs,
 
              # Run test on running server
              'extern=s'                  => \%opts_extern, # Append to hash
@@ -4710,6 +4712,7 @@ sub mysqld_start ($$) {
        nocore        => $opt_skip_core,
        host          => undef,
        shutdown      => sub { mysqld_stop($mysqld) },
+       envs          => \@opt_mysqld_envs,
       );
     mtr_verbose("Started $mysqld->{proc}");
   }
@@ -5724,9 +5727,10 @@ Options for test case authoring
   check-testcases       Check testcases for sideeffects
   mark-progress         Log line number and elapsed time to <testname>.progress
 
-Options that pass on options
+Options that pass on options (these may be repeated)
 
   mysqld=ARGS           Specify additional arguments to "mysqld"
+  mysqld-env=VAR=VAL    Specify additional environment settings for "mysqld"
 
 Options to run test on running server