Default test list

[sampsyo]

It would be cool to be able to just run turnt with no arguments to run a pre-specified list of tests. I propose that turnt.toml gain a tests option, which can either be a string containing glob patterns:

tests = "*.t more/*.t"

Or a list of specific (non-glob) filenames:

tests = [
  "foo",
  "bar",
]

One complication to implementing this is that turnt.toml loading is currently per-test, i.e., each test gets its own configuration from wherever it happens to be located. This will require (also) loading a configuration from the working directory. We'll have to decide how to combine the two configurations—merge them, ignore one or the other, or something else.