[Tests] build tests in test env

Author: ljharb

package.json

@@ -14,7 +14,7 @@
     "prepublishOnly": "npm run build",
     "pretest": "npm run lint:fix && npm run flow",
     "test": "npm run tests-only",
-    "build:tests": "npm run build && rimraf __tests-built__ && babel __tests__ --out-dir __tests-built__",
+    "build:tests": "npm run build && rimraf __tests-built__ && BABEL_ENV=test babel __tests__ --out-dir __tests-built__",
     "tests-built": "nyc tape '__tests-built__/**/*.js'",
     "tests-only": "nyc tape -r @babel/register '__tests__/**/*.js'",
     "prepack": "npm run build"