bpo-36611: Fix test_sys.test_getallocatedblocks() (GH-12797)

Fix test_sys.test_getallocatedblocks() when tracemalloc is enabled.
If the name of Python memory allocators cannot get read, consider
that pymalloc is disabled.

Fix the following error:

./python -X tracemalloc -m test test_sys -v -m test_getallocatedblocks

ERROR: test_getallocatedblocks (test.test_sys.SysModuleTest)
------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_sys.py", line 770, in test_getallocatedblocks
    alloc_name = _testcapi.pymem_getallocatorsname()
RuntimeError: cannot get allocators name

Author: vstinner

Lib/test/test_sys.py

@@ -767,8 +767,13 @@ def test_getallocatedblocks(self):
         except ImportError:
             with_pymalloc = support.with_pymalloc()
         else:
-            alloc_name = _testcapi.pymem_getallocatorsname()
-            with_pymalloc = (alloc_name in ('pymalloc', 'pymalloc_debug'))
+            try:
+                alloc_name = _testcapi.pymem_getallocatorsname()
+            except RuntimeError as exc:
+                # "cannot get allocators name" (ex: tracemalloc is used)
+                with_pymalloc = True
+            else:
+                with_pymalloc = (alloc_name in ('pymalloc', 'pymalloc_debug'))
 
         # Some sanity checks
         a = sys.getallocatedblocks()

Misc/NEWS.d/next/Tests/2019-04-12-12-44-42.bpo-36611.UtorXL.rst

@@ -0,0 +1,2 @@
+Fix ``test_sys.test_getallocatedblocks()`` when :mod:`tracemalloc` is
+enabled.