Testsuite fixes for ksh. * tests/check10.test: Add ':' as last command in subshell, for zero exit status of the subshell. * tests/parallel-am.test: Do not let failing 'unset' of nonexistent variable exit the test. * tests/parallel-am2.test: Likewise. * tests/parallel-am3.test: Likewise. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>