75170 Replace context FakeUsage with TestUsage (cla: yes, team, tool)

75171 Adopt tool_test shard builders (cla: yes, team, 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)

75264 Re-land “Migrate driver tests in example/ to NNBD (#75022)” (cla: yes, d: examples, team, waiting for tree to go green)