Fixes#5437
I don't think changing the behavior was quite the correct thing to do. This new behavior is very similar to the old with a few small differences:
```
cargo test
ORGINAL: Only builds examples.
NEW: Builds all examples. Any example with `test` set is tested.
cargo test --tests
ORIGINAL: Runs all examples as tests.
NEW: Only runs examples as tests if `test` is set.
cargo test --examples
ORIGINAL: Runs all examples as tests.
NEW: (SAME)
cargo test --example foo
ORIGINAL: Runs the given example as a test.
NEW: (SAME)
cargo test --all-targets
ORIGINAL: Runs all examples as tests.
NEW: (SAME)
```