The RP2040 has 2 cores and supports running at most 2 Python threads (the
main one plus another), and will raise OSError if a thread cannot be
created because core1 is already in use. This commit adjusts some thread
tests to be robust against such OSError's. These tests now pass on rp2
boards.
Signed-off-by: Damien George <damien@micropython.org>