76264 MockProcessManager -> FakeProcessManager in os_test (cla: yes, team, tool, waiting for tree to go green)

76266 Swap Mocks to Fakes in build_fuchsia_test (cla: yes, team, tool, waiting for tree to go green)

76268 [flutter_tools] replace some mock file/directories with new op handle (cla: yes, tool, waiting for tree to go green)

76274 Swap Mocks to Fakes in config_test and other tests (cla: yes, team, tool, waiting for tree to go green)

76276 Adopt FakeProcess, remove MockProcess (cla: yes, team, tool, waiting for tree to go green)

76277 [flutter_tools] remove Artifacts dependency from globals in gradle, remove MockArtifacts from test (cla: yes, tool)