Test harness is injected on --cfg test

[alexcrichton]

Cargo has an issue (rust-lang/cargo#2305) with a proposed fix (rust-lang/cargo#2469), but it unfortunately can't land because the compiler currently considers --test and --cfg test synonymous.

I'm... surprised by this behavior, and I'd expect --cfg test to only pass the test cfg.

[alexcrichton]

Ah, and the steps to reproduce:

$ touch bar.rs
$ rustc --cfg test bar.rs
$ ./bar

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

[sanxiyn]

I looked at the history. Originally, test harness was only injected for --test, not for --cfg test. Then b808eab (in 2012!) explicitly changed this, with comments (but without rationale...). Fix seems to be a matter of reverting that commit.