Run test suite with `ctest`

[joewallwork]

Closes #249.

Tidies up the test suite to just run with ctest directly, rather than navigating into subdirectories.

A big advantage of this approach is that we can use the various useful arguments for ctest, such as ctest -R unittest_tensor_autograd to run that specific test.

A caveat is that I needed to make the pip install -r requirements an initial ctest test for each example.

Questions for discussion

• [x] How about we just remove the run_test_suite.sh script and recommend that users call ctest in the build directory to run the tests?
  • We decided it should indeed be removed.

Checklist for author

• [ ] Test on Windows and determine whether the run_test_suite.bat script should also be dropped.