tfragment_alloc.nim (which allocates 4GB) often makes appveyor fail with out of memory

[timotheecour]

@Araq this commit 6d4107b allocates 4GB

  when defined(cpu64):
    # bug #7120
    var x = alloc(((1 shl 29) - 4) * 8)
    dealloc x

and often causes appveyor to fail with out of memory, eg:
https://ci.appveyor.com/project/Araq/nim/build/5348

PASS: tunhandledexc.nim C                                          (1.73437524 secs)
PASS: twrongexc.nim C                                              (1.63440394 secs)
tests\testament\tester.exe "--nim:compiler\nim " cat fragmentation
FAIL: tfragment_alloc.nim C
Test "tests\fragmentation\tfragment_alloc.nim" in category "fragmentation"
Failure: reExitcodesDiffer
Expected:
exitcode: 0
Gotten:
exitcode: 1
Output:
out of memory
SKIP: tfragment_gc.nim C
FAILURE! total: 2 passed: 0 skipped: 1

(I've seen this flaky test a number of times)

Should we disable this test?

NOTE to self:
((1 shl 29) - 4) * 8 is 4GB