ios: Add --no-build flag to run_unittests.py

Introduces a --no-build flag to run_unittests.py which skips compiling/
building the tests and runs them instantly on the simulator. This avoids
unnecessary 5-10 second compilation checks when the binary has already
been built.

- Adds --no-build argument parser option to run_unittests.py.
- Skips _build_tests() execution if set.
- Documents the new --no-build flag inside ios.md prompt template.

Bug: 495937056
Change-Id: I4e169558a0ee7293d9284f7dec20af606d8b4d77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7852736
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Scott Yoder <scottyoder@google.com>
Auto-Submit: Justin Cohen <justincohen@google.com>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@google.com>
Cr-Commit-Position: refs/heads/main@{#1631301}