Improve export blocks image test

[mjgallag]

General items:

• I have updated the relevant documentation files under docs/
• My code follows the:
  • Google Java style guide (for .java files)
  • Google JavaScript style guide (for .js files)
• ant tests passes on my machine

If your code changes how something works on the device (i.e., it affects the companion):

• I branched from ucr
• My pull request has ucr as the base

Further, if you've changed the blocks language or another user-facing designer/blocks API (added a SimpleProperty, etc.):

• I have updated the corresponding version number in appinventor/components/src/.../common/YaVersion.java
• I have updated the corresponding upgrader in appinventor/appengine/src/.../client/youngandroid/YoungAndroidFormUpgrader.java (components only)
• I have updated the corresponding entries in appinventor/blocklyeditor/src/versioning.js

For all other changes:

• I branched from master
• My pull request has master as the base

What does this PR accomplish?

Description

Fixes export test download leak and uses act() helper in export blocks image test.