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

.azure-pipelines.yml

@@ -58,7 +58,7 @@ jobs:
       displayName: Cargo build release
     - script: cargo build
       displayName: Cargo build
-    - bash: RUST_BACKTRACE=1 cargo test --all
+    - bash: RUST_BACKTRACE=1 cargo test --features wasm_tests --all
       displayName: Cargo test
     - script: cargo doc
       condition: eq(variables['Agent.OS'], 'Darwin')