[REQUIRED] Describe your environment

• Operating System version: macOS Catalina 10.15.3
• Browser version: n/a
• Firebase SDK version: HEAD
• Firebase Product: firestore

[REQUIRED] Describe the problem

Steps to reproduce:

In commit 2919c0a, the files spec_builder.ts and spec_test_runner.ts were modified to use base64-encoded resume tokens. Although this makes the tests consistent with the real implementation, it has two undesirable side effects: (1) When tests are exported to Android and iOS via generate_spec_json.sh the resume tokens in the generated JSON files are the base64-encoded resume tokens and (2) It makes the tests that fail due to resume token mismatches more difficult to debug since the resume token cannot be immediately correlated back to the test steps.

Since resume tokens are just opaque strings, there is no need to base64 encode them for the spec tests.