Use unsupported in 'require'

This changes 'require' to use 'unsupported' rather than 'untested'.
The latter doesn't really seem to be correct according to the DejaGNU
documentation:

    Declares a test was not run.  `untested' writes in the log file a
    message beginning with _UNTESTED_, appending the `message' argument.
    For example, you might use this in a dummy test whose only role is to
    record that a test does not yet exist for some feature.

The example there, and some text elsewhere, is what makes me think
this isn't a great fit.  On the other hand, 'unsupported' says:

    Declares that a test case depends on some facility that does not exist
    in the testing environment.

gdb/testsuite/lib/gdb.exp

@@ -9081,7 +9081,7 @@ proc require { args } {
 	    set fn $arg
 	}
 	if {$ok != !![uplevel 1 $fn]} {
-	    untested "require failed: $arg"
+	    unsupported "require failed: $arg"
 	    return -code return 0
 	}
     }