Add engine tests for Linux shell

[robert-ancell]

Description

Add engine tests for Linux shell

Related Issues

None.

Tests

This PR is adding tests that are now possible due to the FlutterEngineProcTable addition.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the contributor guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the C++, Objective-C, Java style guides for the engine.
• I read the tree hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Reviewer Checklist

• I have submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

Breaking Change

Did any tests fail when you ran them? Please read handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.

[chinmaygarde]

Stuart is out till Monday. @cbracken is taking a look in the meantime.

[stuartmorgan-g]

LGTM