subunit.sh: Properly capture and pass on the command output.

subunit.sh: Properly capture and pass on the command output.

Previously, the output from $cmdline was never captured.  In case of a
failure, there was no output being passed to the subunit_fail_test() function,
but that function contains a call to "cat -".  This caused the script to hang
indefinitely waiting for input.

We now capture $cmdline output (including mapping stderr to stdout) using
backticks, and then pipe that output to the subunit_fail_test() if there is
a failure.
(This used to be commit c0234d13192c1871971b45121249395ef15c5ae5)