[Tests] pre-build the tests, only in CI

Author: ljharb

.github/workflows/test.yml

@@ -13,7 +13,8 @@ jobs:
     with:
       range: '>= 0.4'
       type: majors
-      command: npm run tests-only
+      build-command: npx babel __tests__ --out-dir __tests-built__
+      command: nyc tape '__tests-built__/**/*.js'
 
   flow:
     name: Flow type checking