Testthat helper files

load_all() always executes any file below tests/testthat/ that begins with helper.

.                              
├── ...
└── tests
    ├── testthat
    │   ├── helper.R
    │   ├── helper-blah.R
    │   ├── helper-foo.R    
    │   ├── test-foofy.R
    │   └── (more test files)
    └── testthat.R