[test] Allow self-tests to report exit status when running under Linux

[test] Allow self-tests to report exit status when running under Linux

Allow the return status from an embedded image to propagate out to the
eventual return status from main().  When running under Linux, this
allows the pass/fail result of unit tests to be observable without
having to visually inspect the console output.

Signed-off-by: Michael Brown <mcb30@ipxe.org>