Misc testsuite feature gated (#113)

* Put misc_testsuite behind a feature gate

This PR puts building and generating of misc_testsuite behind
a feature gate "misc_testsuite". This is mainly to allow projects
which pull `wasi-common` as a dependency not to have to have
`wasm32-wasi` target installed in order to build it as it currently
is.

* Update the CI

* Rename feature to wasm_tests

* Explain integration testing in the README

tests/wasm_tests.rs

@@ -0,0 +1,16 @@
+#![cfg(feature = "wasm_tests")]
+
+mod runtime;
+mod utils;
+
+use std::sync::Once;
+
+static LOG_INIT: Once = Once::new();
+
+fn setup_log() {
+    LOG_INIT.call_once(|| {
+        pretty_env_logger::init();
+    })
+}
+
+include!(concat!(env!("OUT_DIR"), "/misc_testsuite_tests.rs"));