parallel-tests: Fix summary output. * lib/am/check.am (am__text_box): Fix unportable sed script, replacing `\n' in the right hand side of an `s' command with a literal newline. Kudos to Bruno Haible for the newline idea. * tests/parallel-tests.test: Update test to expose this. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>