Use node-red-node-test-helper module

[kazuhitoyokoi]

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Current travis setting uses git clone command to prepare Node-RED environment. If the master branch of Node-RED repository is under development and doesn't run correctly, grunt will encounter errors. To solve the problem, I use helper module, node-red-node-test-helper instead of that.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 76.821% when pulling 46f14f3 on kazuhitoyokoi:master-use-helper into c2c18f5 on node-red:master.

[kazuhitoyokoi]

@HiroyasuNishiyama Could you merge this?

[kazuhitoyokoi]

@HiroyasuNishiyama Thank you! 👍