Better control when to run tests with Heroku CI

### Required Terms

- [x] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [x] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku CI

### Tell us about what you're trying to solve. What challenges are you facing?

## Suggestion
Current behaviour described on [Heroku devcenter](https://devcenter.heroku.com/articles/heroku-ci#test-run-behavior):

> After you enable Heroku CI for a pipeline, it runs tests automatically for every subsequent push to your GitHub repository. This means that all GitHub pull requests are automatically tested, along with any merges to master.

This is, in some cases, excellent and desired behaviour. But it is not the best solution for bigger projects that have multiple work-in-progress branches. Running tests on every push to the repository means that tests run when we publish a branch or push a commit to the work-in-progress branch.

Heroku CI would be a great solution for running tests if it worked similarly to Heroku Review Apps, not on every push to the repository, but on every push to the branch that has a pull request opened.

## Example:

Good example: test run for branch `fb/heroku_ci`, commit: `f618363`. The said branch has a pull request opened, and there is a link to it.


Bad example: test run for branch `heroku_support_example`, commit: `ff3263b`. The said branch doesn't have a pull request opened, and I don't see a reason for tests to run for it.