Partially revert change to testing examples.

Partially revert change to testing examples.

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)
```