"calls tick repeatedly" test is flaky.

[jmalloc]

go test ./...
ok  	github.com/dogmatiq/testkit	1.023s
Running Suite: github.com/dogmatiq/testkit/engine_test
======================================================
Random Seed: 1606293184
Will run 18 of 18 specs

••••••••••••••
------------------------------
• Failure [0.017 seconds]
func Run()
/Users/james/grit/github.com/dogmatiq/testkit/engine/run_test.go:16
  calls tick repeatedly [It]
  /Users/james/grit/github.com/dogmatiq/testkit/engine/run_test.go:34

  Expected
      <int>: 4
  to be >=
      <int>: 6

  /Users/james/grit/github.com/dogmatiq/testkit/engine/run_test.go:52
------------------------------
•••

Summarizing 1 Failure:

[Fail] func Run() [It] calls tick repeatedly 
/Users/james/grit/github.com/dogmatiq/testkit/engine/run_test.go:52

Ran 18 of 18 Specs in 0.078 seconds
FAIL! -- 17 Passed | 1 Failed | 0 Pending | 0 Skipped
--- FAIL: TestSuite (0.08s)

This happens occasionally on CI.