Omit test categories when unused.

[kyteware]

Problem

Unused test categories clutter results, making them much less digestible on smaller terminals.

E.g. default output on a repository with a single unit test and no docs or main.rs tests:

cargo test
   Compiling game v0.1.0 (/Users/dworv/Coding/ubiquitous/game)
    Finished test [unoptimized + debuginfo] target(s) in 0.22s
     Running unittests src/lib.rs (target/debug/deps/game-8b0a87aac65c670c)

running 1 test
test tests::server::test ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running unittests src/main.rs (target/debug/deps/game-349d00b1b415c1f8)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests game

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Proposed Solution

Instead of showing these 0-test categories, omit them for a cleaner output:

cargo test
   Compiling game v0.1.0 (/Users/dworv/Coding/ubiquitous/game)
    Finished test [unoptimized + debuginfo] target(s) in 0.22s
     Running unittests src/lib.rs (target/debug/deps/game-8b0a87aac65c670c)

running 1 test
test tests::server::test ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

You could even make debugging easier for people that wonder why tests they add wrong don't show up with an extra line:

2 categories with 0 tests omitted

Notes

No response

[epage]

At a quick glance, this looks like a duplicate of #2832. Closing in favor of that. If I missed something, let us know and we can re-open it if needed!