Use build.rs to generate test cases automatically

Now, test binaries are bundled with the repo, and
just like in CraneStation/wasmtime, the test cases
are generated automatically using build.rs. So all
it takes is to drop a new test binary in the
testsuite dir to get the test case for it generated
(with some caveats to do with handling preopens).

Cargo.toml

@@ -37,6 +37,9 @@ target-lexicon = "0.3.0"
 [patch."https://github.com/CraneStation/wasi-common"]
 wasi-common = { path = "." }
 
+[build-dependencies]
+cfg-if = "0.1.9"
+
 [lib]
 name = "wasi_common"
 crate-type = ["rlib", "staticlib", "cdylib"]