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_exit1.py

@@ -13,8 +13,13 @@ def thread_entry():
     _thread.exit()
 
 
-_thread.start_new_thread(thread_entry, ())
-_thread.start_new_thread(thread_entry, ())
+for i in range(2):
+    while True:
+        try:
+            _thread.start_new_thread(thread_entry, ())
+            break
+        except OSError:
+            pass
 
 # wait for threads to finish
 time.sleep(1)