tests: Only clean builddir if it's not srcdir

When they're the same, I'm cleaning away my source files. Not nice - not
nice at all.

Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>
tests: Use old-fashioned serial test harness

Because ubuntu seemingly (?) doesn't support the newfangled stuff.

Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>