Previously the contents of the file being searched was not output,
making it hard to know how to fix a test so that it would pass.
Before:
```
$ make test-heroku-16
...
testCffi
ASSERT:Expected </tmp/shunit.ayVGAO/tmp/output.6gMs/stdout> to contain <ZZZZZZZ>
testPylibmc
```
After:
```
$ make test-heroku-16
...
testCffi
ASSERT:Expected </tmp/shunit.FXWeuI/tmp/output.7Vfx/stdout> to contain <ZZZZZZZ>
! Warning: Your application is missing a Procfile. This file tells Heroku how to run your application.
! Learn more: https://devcenter.heroku.com/articles/procfile
-----> Installing python-3.6.2
-----> Installing pip
-----> Installing requirements with pip
Collecting cffi (from -r /tmp/testBoLG0/requirements.txt (line 1))
Downloading cffi-1.10.0-cp36-cp36m-manylinux1_x86_64.whl (406kB)
Collecting pycparser (from cffi->-r /tmp/testBoLG0/requirements.txt (line 1))
Downloading pycparser-2.18.tar.gz (245kB)
Installing collected packages: pycparser, cffi
Running setup.py install for pycparser: started
Running setup.py install for pycparser: finished with status 'done'
Successfully installed cffi-1.10.0 pycparser-2.18
testPylibmc
```
In cases where there is no stderr there was previously no output,
which made it hard to find the cause. The assertion messages have
also been adjusted to remove the repetition (shunit2 already outputs
expected vs actual itself).
Before:
```
testPipenv
ASSERT:Expected captured exit code to be 0; was <2> expected:<0> but was:<2>
testPipenvVersion
```
After:
```
testPipenv
ASSERT:Captured exit code - expected:<0> but was:<2>
! Warning: Your application is missing a Procfile. This file tells Heroku how to run your application.
! Learn more: https://devcenter.heroku.com/articles/procfile
-----> Installing python-2.7.13
-----> Installing pip
-----> Generating 'requirements.txt' with pipenv
testPipenvVersion
```
Fixes#389.
Ed Morley
kennethreitz