Import i64 testcases from WASM Spec test

[aborg-dev]

We want to import test cases from https://github.com/WebAssembly/spec/blob/master/test/core/i64.wast to our codebase and start tracking our progress on passing them, ideally as a part of the CI. This should give us more confidence that we correctly captured the semantics of WASM ops for integers.

We should check if wasmtime already imports these tests and try to reuse them, but if not, we can manually import these tests in a format that is compatible with our current test suite.

[aborg-dev]

We do have WASM Spec testsuite pulled in as a submodule:

wasmtime/.gitmodules

Line 1 in 2feb144

[submodule "tests/spec_testsuite"]

So we should aim to reuse it.

[aborg-dev]

As an output, we want a table of passed/failed tests within that spec test.