This commit moves testing of `*.wast` files out of the `all` test suite
binary and into its own separate binary. The motivation for this is
well-described in bytecodealliance#4861 with one of the chief reasons being that if the
test suite is run and then a new file is added re-running the test suite
won't see the file.

The `libtest-mimic` crate provides an easy way of regaining most of the
features of the `libtest` harness such as parallel test execution and
filters, meaning that it's pretty easy to switch everything over. The
only slightly-tricky bit was redoing the filter for whether a test is
ignored or not, but most of the pieces were copied over from the
previous `build.rs` logic.

Closes bytecodealliance#4861