re-order scheme so tests are built/run last.
making reporters-tests run via TEST_AFTER_BUILD=YES

We needed to have a CopyFiles step for OCHamcrest + OCMockito.
show better error when AvailableReporters() fails
when running tests, set the 'XT_INSTALL_ROOT' env var.

This fixes the problem where tests would fail when run from
`make_release.sh`.
only generate code context for exceptions if the named file exists.

This fixes the problem where you have a bunch of the following in your
output --

```
2013-08-27 11:27:04.779 pretty[76400:707] ERROR: couldn't load file:
Unknown.m
```

When SenTestingKit doesn't know where an exception occurred, it just
gives you 'Unknown.m', which doesn't exist, and therefore we shouldn't
try to read from it.