tests/thread: Make exc1,exit1,exit2,stacksize1,start1 tests run on rp2.

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>

tests/thread/thread_stacksize1.py

@@ -41,7 +41,12 @@ n_finished = 0
 # set stack size and spawn a few threads
 _thread.stack_size(sz)
 for i in range(n_thread):
-    _thread.start_new_thread(thread_entry, ())
+    while True:
+        try:
+            _thread.start_new_thread(thread_entry, ())
+            break
+        except OSError:
+            pass
 
 # reset stack size to default (for subsequent scripts on baremetal)
 _thread.stack_size()