Add tests for the wasm-gc crate

This commit adds a test harness and the beginnings of a test suite for
the crate that performs GC over a wasm module. This crate historically
has had zero tests because it was thought that it would no longer be
used once LLD landed with `--gc-sections`, but `wasm-bindgen` has come
to rely more and more on `wasm-gc` for various purposes.

The last release of `wasm-bindgen` was also released with a bug in the
recently refactored support in the `wasm-gc` crate, providing a perfect
time and motivation to start writing some tests!

All tests added here are `*.wat` files which contain the expected output
after the gc pass is executed. Tests are automatically updated with
`BLESS_TESTS=1` in the environment, which is the expected way to
generate the output for each test.

crates/gc/tests/wat/remove-import-function.wat

@@ -0,0 +1,26 @@
+(module
+  (import "" "a" (func $i1))
+  (import "" "b" (func $i2))
+  (import "" "c" (func $i3))
+
+  (func $bar)
+
+  (func $foo
+    call $i1
+    call $i3)
+
+  (func $baz)
+
+  (export "foo" (func $foo))
+  )
+
+;; STDOUT (update this section with `BLESS_TESTS=1` while running tests)
+;; (module
+;;   (type (;0;) (func))
+;;   (import "" "a" (func $i1 (type 0)))
+;;   (import "" "c" (func $i3 (type 0)))
+;;   (func $foo (type 0)
+;;     call $i1
+;;     call $i3)
+;;   (export "foo" (func $foo)))
+;; STDOUT