Add mix test --dry-run flag

[Nezteb]

Aims to address #14393

[whatyouhide]

So, implementation-wise I think we might need to take a different approach. Right now, I see two main issues:

1. We're not running ExUnit.Formatters on dry runs—I think we might want to. Basically, the only thing we want --dry-run to do is to literally skip executing the body of tests (+ setup/setup_all calls and whatnot), but everything else would be the same.
2. We are printing test files, but this doesn't give us information about what tests (or test cases even, if there are >1 test cases in a file) would run. I think this is important. Think of using --dry-run to answer the question "if I pass this --exclude option, how many tests will be skipped"?.

Do these points make sense? I am not even sure this is very feasible, so we'd need to sort of proof-of-concept this.

[Nezteb]

@whatyouhide Hello again! Sorry for the ping; I just wanted to check in occasionally so I don't lose track of this PR; I'm in no actual rush. 😄

[whatyouhide]

@Nezteb I won't be able to review today, but I might find time tomorrow. I put it on my todo list!

[whatyouhide]

I tried this locally and there are some issues.

The biggest one is that it prints Test dry run: for files that then exeute no tests. An example from stream_data:

→ mix test --dry-run
Running ExUnit with seed: 188086, max_cases: 28
Excluding tags: [:stdlib]

Test dry run:
Test dry run:
test/stream_data/lazy_tree_test.exs:16
test/stream_data/lazy_tree_test.exs:6
test/stream_data/lazy_tree_test.exs:77
test/stream_data/lazy_tree_test.exs:6
test/stream_data/lazy_tree_test.exs:8
test/stream_data/lazy_tree_test.exs:68
test/stream_data/lazy_tree_test.exs:52
test/stream_data/lazy_tree_test.exs:6
test/stream_data/lazy_tree_test.exs:6
test/stream_data/lazy_tree_test.exs:6
test/stream_data/lazy_tree_test.exs:35
Test dry run:
Test dry run:
Test dry run:
Test dry run:

I think we should print the Test dry run header only once and then just print all tests. Then, we should likely do some formatting. I’m thinking to use color in the header and call it something like Tests that would be executed:, in :cyan (respecting color settings for Mix).

Thoughts?