Test some more things in CI (#469)

Test some more things in CI (#469)

* Test some more things in CI

- Test all GOOS/GOARCH combinations by looping over "go tool dist list";
this just tests if it compiles.

- Add a Vagrant box to test Debian 6 / Linux 2.6.32; this was adapted
from @horahoradev's patch at #434.

- Update the minimum version requirements: we test Linux 2.6.32 now and
  turns out that's also the minimum version [Go supports] in recent
  releases, so just set it to that.

  Need Go 1.16 for retract in go.mod. I don't know, maybe we can just
  remove the retract? Latest Debian ships with Go 1.15.

  [Go supports]: https://github.com/golang/go/issues/45964

- Test both Go 1.16 and 1.18 (the lowest supported version and newest
  version). I guess we could also test 1.17, but the CI already takes
  somewhat long and I can't recall ever having a situation where an
  intermediate version failed.

- Test macOS 11 and 12; macOS 10.15 will (probably) end support in
  November, so probably not worth supporting this. GitHub will [remove]
  support for this at the end of August.

  [remove]: https://github.blog/changelog/2022-07-20-github-actions-the-macos-10-15-actions-runner-image-is-being-deprecated-and-will-be-removed-by-8-30-22/

- Test OpenBSD, NetBSD.

- Move "lint" to its own YAML file.

Future work:

- Actually run tests for all systems Go supports. Bit pointless right
  now as many of these don't do anything. Currently untested are
  Solaris, illumios, plan9, AIX, Android, iOS, DragonflyBSD, WASM.

  Some of these might be difficult (AIX, iOS, Android), but haven't
  looked in to it. I tried setting up Solaris with the
  vmactions/solaris, but it doesn't seem to have an easy way to install
  Go.

- GitHub only [supports] Windows Server 2019 and 2022; probabably also
  want to test Server 2016, but GitHub dropped support for this. Can
  maybe use AppVeyor(?)

  [supports]: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#choosing-github-hosted-runners

- Could maybe test older versions of BSD, too. Not sure of it's worth
  it.

* Skip TestFsnotifyMultipleOperations on NetBSD

This test doesn't seem to work; I'm sure it passed at least once in the
CI, but locally I can't seem to get it to work.

Fails with:

	=== RUN   TestFsnotifyMultipleOperations
    	integration_test.go:114: event received: "/tmp/fsnotify2194826986/TestFsnotifySeq.testfile": CREATE
    	integration_test.go:114: event received: "/tmp/fsnotify2194826986/TestFsnotifySeq.testfile": WRITE
    	integration_test.go:114: event received: "/tmp/fsnotify2194826986/TestFsnotifySeq.testfile": REMOVE|RENAME
    	integration_test.go:114: event received: "/tmp/fsnotify2194826986/TestFsnotifySeq.testfile": CREATE
    	integration_test.go:186: incorrect number of rename+delete events received after 500 ms (2 vs 1)
    	integration_test.go:114: event received: "/tmp/fsnotify2194826986/TestFsnotifySeq.testfile": REMOVE
    	integration_test.go:114: event received: "/tmp/fsnotify2194826986": REMOVE|WRITE

For reference, this is the output on Linux and OpenBSD (the output is
identical):

	=== RUN   TestFsnotifyMultipleOperations
    	integration_test.go:114: event received: "/tmp/fsnotify989736723/TestFsnotifySeq.testfile": CREATE
    	integration_test.go:114: event received: "/tmp/fsnotify989736723/TestFsnotifySeq.testfile": WRITE
    	integration_test.go:114: event received: "/tmp/fsnotify989736723/TestFsnotifySeq.testfile": RENAME
    	integration_test.go:114: event received: "/tmp/fsnotify989736723/TestFsnotifySeq.testfile": CREATE
    	integration_test.go:190: calling Close()
    	integration_test.go:192: waiting for the event channel to become closed...
    	integration_test.go:195: event channel closed

* Fix "too many open files" on Debian 6, maybe

I guess this started failing after I rebased on main; let's see if a
small sleep works to clean up the file descriptors.