Exit with an error if using --test on binary without unit tests compiled

[Calinou]

This makes it clearer that tests=yes should be used at compile-time. Previously, the engine would run as if the argument wasn't passed, which can be confusing.

I've tested behavior with and without tests and it works as expected – --test is blocked, --testsomething or -test are not.

Preview

$ bin/godot.linuxbsd.editor.x86_64 --test          
ERROR: `--test` was specified on the command line, but this Godot binary was compiled without support for unit tests. Aborting.
To be able to run unit tests, use the `tests=yes` SCons option when compiling Godot.

   at: test_entrypoint (main/main.cpp:852)

[akien-mga]

Another option would be to make TEST_MAIN_OVERRIDE and TEST_MAIN_PARAM_OVERRIDE no-ops when TESTS_ENABLED is off. Maybe a bit cleaner?

[Calinou]

Another option would be to make TEST_MAIN_OVERRIDE and TEST_MAIN_PARAM_OVERRIDE no-ops when TESTS_ENABLED is off. Maybe a bit cleaner?

I've pushed a second commit that attempts this, but it doesn't seem to work when I use --test when tests are enabled (the argument is ignored). The error does work when tests are disabled though.

[akien-mga]

I guess let's stick to the original commit if the other approach doesn't seem to work. It's less code too.

[Calinou]

I guess let's stick to the original commit if the other approach doesn't seem to work. It's less code too.

Done; I've removed the second commit.

[akien-mga]

Thanks!