ktest: Allow a test case to undefine a default value

Allow a test case in the config file to undefine a default
value by specifying the option and equal sign but not assigning
it a value:

  OPTION =
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

tools/testing/ktest/ktest.pl

@@ -97,6 +97,11 @@ sub set_value {
 	die "Error: Option $lvalue defined more than once!\n";
     }
     $opt{$lvalue} = $rvalue;
+    if ($rvalue =~ /^\s*$/) {
+	delete $opt{$lvalue};
+    } else {
+	$opt{$lvalue} = $rvalue;
+    }
 }
 
 sub read_config {