testbench: use log_f() consistently in sched_suite.c

I.e. log_f() in case functions, diag() in tests, no printf() anywhere.
lib: remove "[ERR]: " prefix from prints to stderr

It's not useful, and it makes testbench's TAP output look wonky.
testbench: correct @rs init in Mung::TestResult
testbench: output colorized test log for non-OK tests

Passed test points have a green plus, and failed tests have a bold bright_red
exclamation mark. Thank you based Term::ANSIColor
testbench: fix delay_test pt4 in string/meta

Looked like this test point would never pass when SystemClock was functioning
properly, and the condition was hugely confused also.
testbench: improve report colours

The reporting stuff could be in a different package altogether so that it'll
be easier to test on scripted outputs.