Refactor autocert test setup to be clearer

[kenjenkins]

A few of the autocert tests are structured in an unconventional way, using a map[string]func(t *testing.T) test:

• pomerium/config/autocert_test.go

  Line 66 in a95423b

  tests := map[string]func(t *testing.T) test{

• pomerium/internal/autocert/manager_test.go

  Line 390 in a95423b

  tests := map[string]func(t *testing.T) test{

I think the intention is to allow particular tests to run some initialization steps before some shared test case logic, but I find the implementation confusing, especially how the loop body contains:

pomerium/internal/autocert/manager_test.go

Lines 444 to 445 in a95423b

	tc := run(t)
	t.Run(name, func(t *testing.T) {

The first run() call does not run the test, but instead the (optional) initialization steps. I think we could make it clearer what's going on by renaming/refactoring some of this setup logic.

In addition, by running the initialization in the main test (instead of inside the sub-test), a failure of the initialization steps in one test case may prevent the remaining test cases from running.