waf: Add a custom waf tool `gtester` for testing.

Advantages compared to the inbuilt `waf_unit_test` tool:
* both local `./waf` and system wide `waf` calls work (no more includes required)
* tests are always executed, so potential bugs blink in red/yellow colors
*
ci: Remove an obsolete switch from scripts.
waf: Rename module from `gtester` to `unites`, properly convey long running tests.

Use `ctx.start_msg()` and `ctx.end_msg()` when starting and finishing a test process.
Totally independent from `gtester`.