test-lib.sh: colors to test output when parallel(1) is run on tty

Done via $COLORS_WITHOUT_TTY environment variable as passing options
to commands through parallel(1) does not look trivial.

Reorganized color checking in test-lib.sh a bit for this (perhaps
were not fully necessary but rest still an improvement):

  - color checking commands in subshell are not run before arg parsing
    (args may disable colors with --no-color)

  - [ -t 1 ] is checked before forking subshell