async function onTestDir(/** @type {string} */ rootTestDir) {
      for (const testDir of await fs.readdir(rootTestDir)) {
        if (isIgnoredFile(testDir)) continue

        const schemaPath = path.join(SchemaDir, testDir + '.json')
        if (!(await exists(schemaPath))) {
          printErrorAndExit(new Error(), [
            `Failed to find a schema file at "./${schemaPath}"`,
            `Expected schema file computed from directory at "./${path.join(rootTestDir, testDir)}"`,
          ])
        }
      }
    }
Check that each test file has a corresponding schema. We only need to
check "one way". That is, a schema doesn't need to have any corresponding
positive or negative tests.