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

Cargo.toml

@@ -10,6 +10,11 @@ edition = "2018"
 license = "Apache-2.0 WITH LLVM-exception"
 description = "WASI implementation in Rust"
 
+[features]
+# this feature requires wasm32-wasi target installed, and it enables wasm32
+# integration tests when run with `cargo test --features wasm_tests`
+wasm_tests = []
+
 [dependencies]
 wasi-common-cbindgen = { path = "wasi-common-cbindgen" }
 failure = "0.1"