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.

misc_testsuite/empty.wast

@@ -0,0 +1,3 @@
+(module (func (export "empty")))
+
+(invoke "empty")