tests/thread: Add test for concurrent interning of strings.

Qstr code accesses global state and needs to be made thread safe.

tests/thread/thread_qstr1.py

@@ -0,0 +1,34 @@
+# test concurrent interning of strings
+#
+# MIT license; Copyright (c) 2016 Damien P. George on behalf of Pycom Ltd
+
+import _thread
+
+# function to check the interned string
+def check(s, val):
+    assert type(s) == str
+    assert int(s) == val
+
+# main thread function
+def th(base, n):
+    for i in range(n):
+        # this will intern the string and check it
+        exec("check('%u', %u)" % (base + i, base + i))
+
+    with lock:
+        global n_finished
+        n_finished += 1
+
+lock = _thread.allocate_lock()
+n_thread = 4
+n_finished = 0
+
+# spawn threads
+for i in range(n_thread):
+    _thread.start_new_thread(th, (i * 1000, 1000))
+
+# busy wait for threads to finish
+while n_finished < n_thread:
+    pass
+
+print('pass')