Fix feature-gating of test-programs (#612)

* Fix feature-gating of test-programs

This commit fixes bugs in enabling feature-gating of `test-programs`
which was introduced in #600. It turns out, #600 accidentally
disabled `test-programs` from ever running, and this commit fixes
that.

* Fix the CI

crates/wasi-common/README.md

@@ -60,10 +60,10 @@ rustup target add wasm32-wasi
 [rustup]: https://rustup.rs
 
 Now, you should be able to run the integration testsuite by running `cargo test` on the
-`test-programs` package:
+`test-programs` package with `test_programs` feature enabled:
 
 ```
-cargo test --package test-programs
+cargo test --features test_programs --package test-programs
 ```
 
 ## Third-Party Code