oeqa.buildperf: make tests independent

oeqa.buildperf: make tests independent

Add test set-up functionality so that the individual tests do not depend
on each other. This should make sure that a failure in one test does not
affect the results of another test. The patch also makes it reasonable
to run only a subset of the tests by using the --run-tests option.

The increase in total execution time of the full suite - caused by the
additional set-up steps - is insignificant because normally no
additional tasks need to be run. The previous test has already done all
set-up work.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>