don't use LaunchTaskAndCaptureOutputInCombinedStream to query build settings

The problem is that xcodebuild will occasionally spew NSLog messages on
STDERR, which will break the parser.  Let's keep the streams separate.
in OtestShimTests, add functions for interacting with OSX tests.
in OTestShimTests, don't assert on expected number of tests.

We're about to add more events to the stream, and these tests don't need
to be this specific.
when tests fail to start, include any output emitted by the test runner.

When test bundles crash on startup, there's often useful information
(maybe exception info) printed to STDERR before the test bundle crashes.
Let's capture that output and display it in the failure message.

It will look like ...

```
  run-test TestThatThrowsExceptionOnStart.octest (macosx10.9, logic-test, GC OFF)
      -[FAILED_BEFORE TESTS_RAN]
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Test did not run: the test bundle stopped running or crashed before the
test suite started.

2013-11-22 11:23:32.105 otest[22683:303] *** Terminating app due to
uncaught exception 'NSGenericException', reason: 'Let's crash on start!'
*** First throw call stack:
(
0   CoreFoundation                      0x00007fff95f5e41c __exceptionPreprocess + 172
1   libobjc.A.dylib                     0x00007fff89ec9e75 objc_exception_throw + 43
2   CoreFoundation                      0x00007fff95f5e2cc +[NSException raise:format:] + 204
3   TestThatThrowsExceptionOnStart      0x0000000101bb0c83 Initializer + 115
4   ???                                 0x00007fff65991c2e 0x0 + 140734897921070
5   ???                                 0x00007fff65991dba 0x0 + 140734897921070921466
6   ???                                 0x00007fff6598ea62 0x0  + 140734897908322
7   ???                                 0x00007fff6598e8f6 0x0 + 140734897907958
8   ???                                 0x0x00007fff684b0e 0x0 + 140734897867534
9   ???                                 0x00007fff6598b7cf 0x0 + 140734897895375
10  libdyld.dylib                       0x00007fff960757ef dlopen + 59
11  CoreFoundation                      0x00007fff95ea7e1a _CFBundleDlfcnLoadBundle + 170
12  CoreFoundation                      0x00007fff95ea763b _CFBundleLoadExecutableAndReturnError + 683
13  Foundation                          0x00007fff9121cdd0 _NSBundleLoadCode + 548
14  Foundation                          0x00007fff9121c9bb -[NSBundle loadAndReturnError:] + 259
15  otest                               0x0000000101b13dec otest + 7660
16  otest                               0x0000000101b141a1 otest + 8609
17  otestt                              0x0000000101b13b46 otest + 6982
18  libdyldbdyld.dylib                  0x00007fff960765fd start + 1
19  ???19                               0x000000000000000a 0x0 + 10
)
libc++abi.dylib: terminating with uncaught exception of type NSException
━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
    ☂ -[FAILED_BEFORE TESTS_RAN] (0 ms) (0)
```
only enable the interposed write() functions after the initializer runs.

This fixes a crash bug I've been hitting, where libSystem's intializer
ends up calling into our interposed functions.