golang: tests: Rework verifying blockforever

- put the logic to test-tweak what happens inside _blockforever into
  context manager pypanicWhenBlocked;
- place this manager in pyx code, where it can later be changed to tweak
  _blockforever at C level.

golang/_golang.pyx

@@ -617,7 +617,10 @@ def pyselect(*casev):
 
 
 # _blockforever blocks current goroutine forever.
+_tblockforever = None
 def _blockforever():
+    if _tblockforever is not None:
+        _tblockforever()
     # take a lock twice. It will forever block on the second lock attempt.
     # Under gevent, similarly to Go, this raises "LoopExit: This operation
     # would block forever", if there are no other greenlets scheduled to be run.

golang/_golang_test.pyx

@@ -28,6 +28,7 @@ from __future__ import print_function, absolute_import
 
 from golang cimport go, pychan, panic, pypanic, topyexc
 from golang import nilchan
+from golang import _golang
 
 from golang import time
 
@@ -69,6 +70,21 @@ def pywaitBlocked(pychanop):
         time.sleep(0)   # yield to another thread / coroutine
 
 
+# `with pypanicWhenBlocked` hooks into _golang._blockforever to raise panic with
+# "t: blocks forever" instead of blocking.
+cdef class pypanicWhenBlocked:
+    def __enter__(t):
+        assert _golang._tblockforever is None
+        _golang._tblockforever = _panicblocked
+        return t
+
+    def __exit__(t, typ, val, tb):
+        _golang._tblockforever = None
+
+def _panicblocked():
+    pypanic("t: blocks forever")
+
+
 # small test to verify pyx(nogil) go.
 cdef void _test_go_nogil() nogil except +topyexc:
     go(_work, 111)

golang/golang_test.py

@@ -29,7 +29,7 @@ from six.moves import range as xrange
 import gc, weakref
 
 from golang._golang_test import pywaitBlocked as waitBlocked, pylen_recvq as len_recvq, \
-        pylen_sendq as len_sendq
+        pylen_sendq as len_sendq, pypanicWhenBlocked as panicWhenBlocked
 
 # pyx/c/c++ tests -> test_pyx_*
 from golang import _golang_test
@@ -653,14 +653,8 @@ def bench_select(b):
 
 
 def test_blockforever():
-    from golang import _golang
-    B = _golang._blockforever
-    def _(): panic("t: blocks forever")
-    _golang._blockforever = _
-    try:
+    with panicWhenBlocked():
         _test_blockforever()
-    finally:
-        _golang._blockforever = B
 
 def _test_blockforever():
     z = nilchan