When sub_test fails to parse an integer, write error message and exit cleanly.

This prevents start_test from also reporting that sub_test did not exit
cleanly. For example, previously on systems that don't clone symlinks
correctly, we would see the following two errors (both for the same issue):

```
[Error (sub_test): Invalid integer value in m-lsms.par-forall.numlocales (studies/lsms/shemmy)]
[Error running sub_test in /var/tmp/jenkins/chapel-ci/workspace/correctness-test-linux32-legacy-git/test/studies/lsms/shemmy (255)]
```

Now, only the first error will be reported. We _may_ want to consider making
clean_exit=True the default behavior for the Fatal() function in sub_test.
Update sub_test to exit with 173 when Fatal() is called.

Update start_test to recognize 173 as a special code that means sub_test has
already printed an error message.

173 is the sum of the ascii characters in "CHAPEL", modulo 256 (to be inside
the valid range of exit codes).
Merge branch 'master' of https://github.com/chapel-lang/chapel into sub-test-improvements
Remove clean_exit arg.
Update single file code path in start_test to recognize 173 exit code.
Merge pull request #924 from thomasvandoren/sub-test-improvements

When sub_test fails to parse an integer, write error message and exit cleanly.

This prevents start_test from also reporting that sub_test did not exit
cleanly. For example, previously on systems that don't clone symlinks
correctly, we would see the following two errors (both for the same issue):

```
[Error (sub_test): Invalid integer value in m-lsms.par-forall.numlocales (studies/lsms/shemmy)]
[Error running sub_test in /var/tmp/jenkins/chapel-ci/workspace/correctness-test-linux32-legacy-git/test/studies/lsms/shemmy (255)]
```

Now, only the first error will be reported. sub_test will have this
behavior for any Fatal() calls.

r: @ronawho