Reorganize tests.

- Move spec_testsuite and misc_testsuite under the tests directory.
 - Remove some redundant tests from filetests.
 - Move wat tests to wat/tests.

scripts/test-all.sh

@@ -6,7 +6,7 @@ set -euo pipefail
 # - Check code formatting.
 # - Make a debug build.
 # - Make a release build.
-# - Run unit tests for all Rust crates (including the filetests)
+# - Run unit tests for all Rust crates
 # - Build API documentation.
 # - Optionally, run fuzzing.
 #