finish forking: make sure most test pass w/o errors

[bassosimone]

There is just a test that does not pass at the moment:

> go test ./...                                                   finish-forking
PASS
Too many goroutines running after net/http test(s).
1 instances of:
github.com/ooni/oohttp_test.interestingGoroutines(0x14015d9a1b0, 0x140156ea180, 0x140156f2017)
	/Users/sbs/src/github.com/ooni/oohttp/main_test.go:33 +0x5c
github.com/ooni/oohttp_test.goroutineLeaked(0x1400016a000)
	/Users/sbs/src/github.com/ooni/oohttp/main_test.go:72 +0x11c
github.com/ooni/oohttp_test.TestMain(0x1400016a000)
	/Users/sbs/src/github.com/ooni/oohttp/main_test.go:25 +0x4c
main.main()
	_testmain.go:1153 +0x150
FAIL	github.com/ooni/oohttp	52.947s
ok  	github.com/ooni/oohttp/cgi	(cached)
ok  	github.com/ooni/oohttp/cookiejar	(cached)
ok  	github.com/ooni/oohttp/fcgi	(cached)
ok  	github.com/ooni/oohttp/httptest	(cached)
ok  	github.com/ooni/oohttp/httptrace	(cached)
ok  	github.com/ooni/oohttp/httputil	(cached)
ok  	github.com/ooni/oohttp/internal	(cached)
FAIL

When I did another attempt where I just copied files, I did not see this leak. I suppose this is because here I am using go1.16 as base and there I was using go1.16.5. So, I am going to postpone fixing this issue to after I forward port the patches.