backends: Make ninja not build test executables by default

[dcbaker]

When they're marked build_by_default : false

This soles the easy part, which is making ninja not build test dependencies if they're explicitly marked build_by_default : false. A completely fix will require some deeper changes to not affect backwards compatibility, as currently all targets default to build_by_default=True.

This shouldn't affect the VSCode backend since tests are generated separately, but I could be wrong.

I'm thinking moving to a tri-state value equivalent to Always, If-not-test, never.

Fixes #2518
Fixes #1704

[dcbaker]

Also still needs to have some tests. I've stared doing that but haven't finished yet.

[NNemec]

Please have a look at my PR #5728 - this appears to address exactly the same issue...

[jpakkane]

It would be better if we could do without a tristate value. We need to check for people using not-built executables for tests and error out or print a warning anyway. Failing silently in that case is just bad UX.

So something like:

• if a test is marked as build_by_default: false it will not be built with "ninja all"
• however it will be built with "ninja test"

[dcbaker]

@jpakkane, that's exactly what I have now. If we were building this from scratch what I'd really want is build_by_default : false is the default, install : true overrides build_by_default, targets needed by tests would be pulled in during ninja test time. That would break backwards compatibility though so I don't think we can do that.

[dcbaker]

I'll go ahead and close this and take ownership of getting #5728 merged.