Move towards "test the exact release artifacts" in CI

[alexcrichton]

In yesterday's Wasmtime meeting we had a discussion about testing, CI, and the artifacts that we produce. Currently the actual released artifacts are not actually tested in CI at all, they're simply produced. This isn't a great situation and ideally we would test the exact artifacts that CI produces rather than a different build configuration.

There's a few challenges to tackle when handling this though:

• Most of the test suite is not architected to run the wasmtime CLI or the libwasmtime.so C API. Some of tests/all/cli_tests.rs runs the wasmtime executable, but the executable is produced as part of cargo test and isn't a released artifact. The C API is only tested through the C examples (so very lightly).
• Testing artifacts might require a different command to execute or new infrastructure other than cargo test. It's unclear what this infrastructure would look like.
• Testing "all possible configurations" isn't feasible, even with the generous concurrency that we already have. For example we can't test the matrix of {platform, opt/debug, debug-asserts/no, crate} in CI as that would be too much.

We didn't reach many firm conclusions in the meeting but I wanted to open a follow-up issue to track further progress. A few points we did identify were:

• We should probably have one CI job test with --release to test with optimizations (just one platform though, probably not all of them)
• We should ideally run at least a "smoke test" for released artifacts.

cc @tschneidereit