MDEV-12785 MTR complains "Can't exec mariadb_config"

Adjust search paths for mariadb_config and make further assignment
conditional

mysql-test/mysql-test-run.pl

@@ -2280,10 +2280,15 @@ sub environment_setup {
   $ENV{'MYSQL_EMBEDDED'}=           $exe_mysql_embedded;
 
   my $client_config_exe=
-    native_path("$bindir/libmariadb/mariadb_config$opt_vs_config/mariadb_config");
+    mtr_exe_maybe_exists(
+        "$bindir/libmariadb/mariadb_config$opt_vs_config/mariadb_config",
+               "$bindir/bin/mariadb_config");
+  if ($client_config_exe)
+  {
     my $tls_info= `$client_config_exe --tlsinfo`;
     ($ENV{CLIENT_TLS_LIBRARY},$ENV{CLIENT_TLS_LIBRARY_VERSION})=
       split(/ /, $tls_info, 2);
+  }
   my $exe_mysqld= find_mysqld($basedir);
   $ENV{'MYSQLD'}= $exe_mysqld;
   my $extra_opts= join (" ", @opt_extra_mysqld_opt);