The sole purpose of interactive_ui_tests is to run tests single threaded. Otherwise they could as well be part of browser_tests.

The fact this is two separate executable causes these side effects:
- Longer build time due to additional large executable.
- Longer archival time due to more binaries to archive.
- Obscurity from developers. Many people don't even know why interactive_ui_tests exist.
- One-off solution that doesn't scale to other tests (sbox).
- Irrelevant on linux.
- Slightly more complex BUILD.gn file.
- More complex source file tree, as interactive ui tests must be in separate source files from browser_tests.

Proposal:
- Create a prefix (EXCLUSIVE_ or something else) that signals test launcher that the test case needs to be run serially at the start of the test shard, then run all parallel tests together. The prefix needs to be obvious enough so a new developer can guess correctly.
- Move all test cases in interactive_ui_tests back into browser_tests and rename them with the prefix.
- Stop running interactive_ui_tests.
- Stop building interactive_ui_tests.
- Calculate the savings.