Switch back to python test runner (#8717)

build: Switch to Python browser test runner

The build system now uses the run_browser_tests.py script as the
primary runner for browser tests, replacing the temporary shell
script.  

The Python script has been significantly refactored to provide robust
test execution capabilities. It now supports GTest sharding, enabling
efficient parallel execution across CI/CD environments. It also
integrates GTest XML output, ensuring standardized test reporting.
Improvements include better argument parsing, test binary discovery,
and error handling.

Bug: 433354983

Author: linxinan-chops

cobalt/testing/browser_tests/BUILD.gn

@@ -179,10 +179,7 @@ if (is_android) {
 
 copy("cobalt_browsertests_runner_script") {
   testonly = true
-
-  # TODO(433354983): Switch back to the run_browser_tests.py
-  # once the infra supports python.
-  sources = [ "cobalt_browsertests_runner.sh" ]
+  sources = [ "run_browser_tests.py" ]
   outputs = [ "$root_out_dir/cobalt_browsertests_runner" ]
   deps = [ ":cobalt_browsertests" ]
 }