x/build: add MacOS builder for long tests

[ysmolsky]

We should run long tests on all major platforms, not only on Linux. Whenever test is removed from the short mode, it gets neglected because long tests are not run automatically.

#26473 might move even more tests to the long mode.

[bradfitz]

We have limited numbers of physical Macs, so this might be too expensive.

[bradfitz]

This is easy to do but we shouldn't do it until we have #19178 done (a scheduler for prioritizing builds). Then we can do this at low priority.