Reorganize tests (#523)

* Refactor Lightbeam's tests.

This refactors Lightbeam's tests.rs file into several pieces, separating
quickcheck tests into their own file, and moving tests which can be run as
wast tests into `tests/misc_testsuite`, and creating a tests directory
for the rest.

* Remove the old filetests tests.

These are all covered by misc_testsuite and spec_testsuite tests.

* rustfmt

* Remove the "bench" feature.

crates/lightbeam/Cargo.toml

@@ -30,6 +30,3 @@ quickcheck = "0.9.0"
 
 [badges]
 maintenance = { status = "experimental" }
-
-[features]
-bench = []