This does not happen on tests run with '-R 22:22' or a lower run count, but occur systematically with '-R 23:23' or a greater run count.

$ ./python
Python 3.5.0a0 (default:1e1c6e306eb4, Oct  9 2014, 19:52:59)
[GCC 4.9.1 20140903 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import platform; platform.uname()
uname_result(system='Linux', node='bilboquet', release='3.16.3-1-ARCH', version='#1 SMP PREEMPT Wed Sep 17 21:54:13 CEST 2014', machine='x86_64', processor='')
>>> 

$ ./python -m test -R 23:23 test_capi
[1/1] test_capi
beginning 46 repetitions
1234567890123456789012345678901234567890123456
..............................................
1 test OK.
Debug memory block at address p=0x982570: API ''
    18446744073709551615 bytes originally requested
    The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfb):
        at p-7: 0x00 *** OUCH
        at p-6: 0x00 *** OUCH
        at p-5: 0x00 *** OUCH
        at p-4: 0x00 *** OUCH
        at p-3: 0x00 *** OUCH
        at p-2: 0x00 *** OUCH
        at p-1: 0x00 *** OUCH
    Because memory is corrupted at the start, the count of bytes requested
       may be bogus, and checking the trailing pad bytes may segfault.
    The 8 pad bytes at tail=0x98256f are not all FORBIDDENBYTE (0xfb):
        at tail+0: 0x00 *** OUCH
        at tail+1: 0x00 *** OUCH
        at tail+2: 0x00 *** OUCH
        at tail+3: 0x00 *** OUCH
        at tail+4: 0x00 *** OUCH
        at tail+5: 0x00 *** OUCH
        at tail+6: 0x00 *** OUCH
        at tail+7: 0x00 *** OUCH
    The block was made by call #0 to debug malloc/realloc.
    Data at p:
Fatal Python error: bad ID: Allocated using API '', verified using API 'o'

Current thread 0x00007f525bcf2700 (most recent call first):
Aborted (core dumped)