tests: test CMP0053 in WARN mode when watching variables

When CMP0053 is in WARN mode, variables get expanded twice, leaking the
fact that the string was expanded twice and changing behavior. Instead,
suppress variable watches when running the expansion to trigger the
CMP0053 warning.
Merge topic 'dev/CMP0053-variable_watch' into next

9ba91463 tests: test CMP0053 in WARN mode when watching variables