Usage:

   A)
      v test folder/ : run all v tests in the given folder.
      v -stats test folder/ : the same, but print more stats.
   B)
      v test file_test.v : run test functions in a given test file.
      v -stats test file_test.v : as above, but with more stats.

NB: you can also give many and mixed folder/ file_test.v arguments after test.

NB 2: very frequently, when you work on a module, you can cd into its folder,
and then you can just run:
      v test .
... which will run all the module _test.v files.


NB 3: V builtin testing requires you to name your files with a _test.v
suffix, and to name your test functions with test_ prefix. Each test_
function in a _test.v file will be called automatically by the test
framework. You can use `assert condition` inside each test_ function.
If the asserted condition fails, then v will record that and produce a
more detailed error message about where the failure was.

After all test_ functions are called, if you passed -stats, v will 
produce a report about how many tests passed, and how many failed.