tests: Fix double expansion bugs in test code

Two test programs - check_path and overlay - define a CHECK() helper macro
in such a way that in the case of an error it will re-execute the checked
code fragment, instead of using the return value from the initial call.

This can lead to misreporting errors, because the code may fail in a
different way the second time around due to changes made during the first
failing call.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>