testsdk: allow user to specify which tests to run

testsdk: allow user to specify which tests to run

Following the usage of TEST_SUITES in testimage, add TESTSDK_SUITES to
specify the list of tests to execute. By default the variable is empty,
which means to run all discovered tests.

This makes it easier to work on a single test without having to run all
of the tests.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>