Feature-gate test-programs

This commit feature-gates `test-programs` as a workaround for
`cargo test` not allowing for optional dev-dependencies.

This commit addresses #595.

crates/test-programs/tests/wasm_tests/main.rs

@@ -1,3 +1,4 @@
+#![cfg(features = "test-programs")]
 mod runtime;
 mod utils;