Feature/improved tests

[DavisVaughan]

This PR improves the test suite of xtensor to use testthat in a more standard fashion. Specifically, a few of the changes are:

• The standard testthat structure is used. R test files are now in tests/testthat/<testfile.R>.
• The cpp test helpers have been modularized and placed in src/. They are not exported when the user calls library(xtensor). i.e. the user will not see them with xtensor::, only with xtensor:::. This should be pretty nonintrusive.
• The test/ folder has been removed.
• Travis was updated to no longer try and run the tests in test/. Now tests are automatically run when R CMD check is called. https://travis-ci.org/DavisVaughan/Xtensor.R/jobs/473077087#L1690
• I committed the RcppExports files (R and cpp versions). This is pretty standard

A bit more on the new test structure. I talked with some of my RStudio folks on the best way to have cpp files that only get run during tests, and then called from the R test files. I was encouraged to just place them in src/ and use the Rcpp::export attribute which makes them callable from R for the tests, but not to the general R user who uses the package. At first I tried placing the cpp files in tests/testthat/<test.cpp> and that worked with devtools::check() but not devtools::test() because of some include path problems. All in all, placing them in src/ might seem annoying at the surface level, but I'm convinced it is ultimately the most stable way to do this.

I also added the header folders to the gitignore because they kept popping up in RStudio as suggestions to commit. Hopefully that's fine.

[SylvainCorlay]

Hey @DavisVaughan thanks a lot for this. I am really glad we don't need to expose test functions. This is much better.

I have left a comment regarding the files generated by Rcpp which I think can be added to .gitignore since they are generated with the configure script.

[DavisVaughan]

Should be good to go now

[SylvainCorlay]

Thanks!