selftests: Line buffer test program's stdout

The selftests runner pipes the test program's stdout to tap_prefix. The
presence of the pipe means that the test program sets its stdout to be
fully buffered (as aposed to line buffered when directly connected to
the terminal). The block buffering means that there is often content in
the buffer at fork() time, which causes the output to end up duplicated.
This was causing problems for mm:cow where test results were duplicated
20-30x.

Solve this by using `stdbuf`, when available to force the test program
to use line buffered mode. This means previously printf'ed results are
flushed out of the program before any fork().

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>