QProcess truncates output from a crashing test

The testlib self-test runs each sub-test in a QProcess which collects the output for comparison with expected versions of the output; in a few cases, there are known variants on the output, so several expected output files are provided. The expected output files live in qtbase/tests/auto/testlib/selftests/.

One test case is the crashes test which, as its name suggests, crashes (it deliberately dereferences a nullptr as array, asking for its entry at index [1]). One of its example output files, expected_crashes_4.txt, is a truncation of another, expected_crashes_3.txt, leaving off the last twolines (the test summary details). On attempting to merge qtbase's 5.13 branch into dev, we have run into failures for this test due to a further truncation of this example output, leaving off another two lines.

The truncation only happens on the (Ubuntu) Coin VM, with -no-widgets -force-debug-info passed to configure, along with a -prefix (i.e. not a developer-build, so make install is needed); and may need -nomake test as well (i.e. you'll need to explicitly run qmake on the testlib/selftests/ component after installing). Even so, when I run the compiled crashes/crashes binary, with QTEST_DISABLE_CORE_DUMP=1 and QTEST_DISABLE_STACK_DUMP=1 set in the environment (as the selftest does), from a shell, I get the full expected_crashes_3.txt - so, apparently, the problem is that QProcess is truncating the output, presumably because it learns of the child's death before it collects all the output from it.