oeqa/selftest: Clean up separate builddir in success case when non-threaded
If oe-selftest is run without -j, the separate build directory "build-st"
isn't cleaned up afterwards. Mirror the behaviour of the -j option to
handle this the same way, only preserve upon failure.
To do this, the remove function needs to be moved to the selftest
context module so that it can be accessed without requiring the
testtools and subunit modules the -j option requires.
A dummy wrapper class is used to wrap the tests and clean up afterwards.
[YOCTO #13953]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
