RunCMake: Expect empty output by default.

Expect tests to specify stderr content if it is present.

Fix the CMP0019 test, which has only been testing the WARN status
until now.  Specify in the CommandLine and FPHSA tests that content
is at least one character.
Merge topic 'RunCMake-stderr-default' into next

c41a1f32 RunCMake: Expect empty output by default.