[lldb][test] Disable image lookup colour test on Mac OS

I think it can work there but we need to correct the CHECK lines.

```
command-image-lookup-color.test:34:11: error: CHECK7: expected string not found in input
          ^
```
https://green.lab.llvm.org/green/view/LLDB/job/as-lldb-cmake/10880/testReport/

I don't have a way to see the full output.

Author: DavidSpickett

lldb/test/Shell/Commands/command-image-lookup-color.test

@@ -3,6 +3,9 @@
 # The file main.c is does not show up in search on Windows.
 # UNSUPPORTED: system-windows
 
+# Until we figure out the correct CHECK lines.
+# UNSUPPORTED: system-darwin
+
 # Checking simple regex search
 
 # RUN: %lldb %t -b -o 'settings set use-color true' -o 'image lookup -r -s ma' | FileCheck %s --check-prefix CHECK1