test: run a subset of tests using `libc-test` (#346)

test: run a subset of tests using `libc-test` (#346)

* test: run a subset of tests from `libc-test`

This change introduces a `test` directory that retrieves the `libc-test`
suite, compiles a subset of the tests using `wasi-libc`, and runs them
with Wasmtime.

* ci: run tests during CI

This change includes some fixups to the filesystem to place Clang's
runtime library for `wasm32-wasi` in the right location. Note that this
CI action is limited to a single OS--Linux.