75171 Adopt tool_test shard builders (cla: yes, team, waiting for tree to go green)

75175 Make flutter_driver be usable from a null-safe app (a: tests, cla: yes, framework, waiting for tree to go green)

75179 Adopt subshard naming convention in build_tests (cla: yes, team, team: infra, waiting for tree to go green)

75185 Remove dead tool_coverage code (cla: yes, team)

75189 Remove hard-coded subshard numbers from the test runner (cla: yes, team, waiting for tree to go green)

75193 Update to latest templates (cla: yes, team, waiting for tree to go green)

75194 Remove unactionable message that was appearing during test execution. (cla: yes, framework, waiting for tree to go green)