Bug #44979 Enhance MTR --experimental to support platform qualifier

Adding @<platform> syntax

mysql-test/collections/README.experimental

@@ -23,3 +23,10 @@ The syntax is as follows:
    start with the same characters up to the last letter before the asterisk
    are considered experimental:
         main.a*  # get rid of main.alias, main.alibaba and main.agliolio
+
+6) Optionally, the test case may be followed by one or more platform
+   qualifiers beginning with @ or @!. The test will then be considered
+   experimental only/except on that platform. Basic OS names as
+   reported by $^O in Perl, or 'windows' are supported, this includes
+   solaris, linux, windows, aix, darwin, ...  Example:
+        main.alias  @aix @windows    # Fails on those

mysql-test/collections/default.experimental

@@ -1 +1,2 @@
 funcs_1.charset_collation_1              # depends on compile-time decisions
+main.plugin_load  @solaris               # Bug #42144

mysql-test/mysql-test-run.pl

@@ -984,6 +984,9 @@ sub command_line_setup {
 
   if ( $opt_experimental )
   {
+    # $^O on Windows considered not generic enough
+    my $plat= (IS_WINDOWS) ? 'windows' : $^O;
+
     # read the list of experimental test cases from the file specified on
     # the command line
     open(FILE, "<", $opt_experimental) or mtr_error("Can't read experimental file: $opt_experimental");
@@ -994,6 +997,15 @@ sub command_line_setup {
       # remove comments (# foo) at the beginning of the line, or after a 
       # blank at the end of the line
       s/( +|^)#.*$//;
+      # If @ platform specifier given, use this entry only if it contains
+      # @<platform> or @!<xxx> where xxx != platform
+      if (/\@.*/)
+      {
+	next if (/\@!$plat/);
+	next unless (/\@$plat/ or /\@!/);
+	# Then remove @ and everything after it
+	s/\@.*$//;
+      }
       # remove whitespace
       s/^ +//;              
       s/ +$//;