tests/thread: Replace busy waiting loops with a loop that sleeps.

Depending on the thread scheduler, a busy-wait loop can hog the CPU and
make the tests very slow.  So convert such loops to loops that have an
explicit sleep, allowing the worker threads to do their job.

tests/thread/stress_heap.py

@@ -3,6 +3,10 @@
 #
 # MIT license; Copyright (c) 2016 Damien P. George on behalf of Pycom Ltd
 
+try:
+    import utime as time
+except ImportError:
+    import time
 import _thread
 
 def last(l):
@@ -37,6 +41,6 @@ n_finished = 0
 for i in range(n_thread):
     _thread.start_new_thread(thread_entry, (10000,))
 
-# busy wait for threads to finish
+# wait for threads to finish
 while n_finished < n_thread:
-    pass
+    time.sleep(1)

tests/thread/thread_lock4.py

@@ -2,6 +2,10 @@
 #
 # MIT license; Copyright (c) 2016 Damien P. George on behalf of Pycom Ltd
 
+try:
+    import utime as time
+except ImportError:
+    import time
 import _thread
 
 def fac(n):
@@ -39,6 +43,7 @@ while True:
     with jobs_lock:
         if len(output) == n_jobs:
             break
+    time.sleep(1)
 
 # sort and print the results
 output.sort(key=lambda x: x[0])

tests/thread/thread_qstr1.py

@@ -2,6 +2,10 @@
 #
 # MIT license; Copyright (c) 2016 Damien P. George on behalf of Pycom Ltd
 
+try:
+    import utime as time
+except ImportError:
+    import time
 import _thread
 
 # function to check the interned string
@@ -28,8 +32,8 @@ n_qstr_per_thread = 100 # make 1000 for a more stressful test (uses more heap)
 for i in range(n_thread):
     _thread.start_new_thread(th, (i * n_qstr_per_thread, n_qstr_per_thread))
 
-# busy wait for threads to finish
+# wait for threads to finish
 while n_finished < n_thread:
-    pass
+    time.sleep(1)
 
 print('pass')