./mtr --skip-not-found

New mtr option --skip-not-found makes it to show not found tests
as skipped

main.a                                   [ skipped ]  not found

(but only if the test was specified with the suite name)
and not error out early with

mysql-test-run: *** ERROR: Could not find 'a' in 'main' suite

This is useful in buildbot, on builders that generate the list
of tests dynamically.

mysql-test/lib/mtr_cases.pm

@@ -154,7 +154,17 @@ sub collect_test_cases ($$$$) {
         {
 	  push (@$cases, @this_case);
 	}
-	else
+	elsif ($::opt_skip_not_found)
+        {
+          push @$cases, My::Test->new
+            (
+             name          => "$sname.$tname",
+             shortname     => $tname,
+             skip          => 1,
+             comment       => 'not found',
+            );
+        }
+        else
 	{
 	  mtr_error("Could not find '$tname' in '$sname' suite");
         }

mysql-test/mysql-test-run.pl

@@ -259,6 +259,7 @@ END
 
 our $opt_fast= 0;
 our $opt_force= 0;
+our $opt_skip_not_found= 0;
 our $opt_mem= $ENV{'MTR_MEM'};
 our $opt_clean_vardir= $ENV{'MTR_CLEAN_VARDIR'};
 
@@ -1158,6 +1159,7 @@ sub command_line_setup {
 
              # Control what test suites or cases to run
              'force+'                   => \$opt_force,
+             'skip-not-found'           => \$opt_skip_not_found,
              'suite|suites=s'           => \$opt_suites,
              'skip-rpl'                 => \&collect_option,
              'skip-test=s'              => \&collect_option,
@@ -5802,6 +5804,8 @@ ($)
                         the execution will continue from the next test file.
                         When specified twice, execution will continue executing
                         the failed test file from the next command.
+  skip-not-found        It is not an error if a test was not found in a
+                        specified test suite. Test will be marked as skipped.
   do-test=PREFIX or REGEX
                         Run test cases which name are prefixed with PREFIX
                         or fulfills REGEX