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).

appveyor.yml

@@ -9,7 +9,5 @@ install:
   - rustc -vV
   - cargo -vV
   - rustup component add rustfmt
-  - cd %APPVEYOR_BUILD_FOLDER%
-  - git submodule update --init --recursive
 build: false
 test_script: test-all.bat