Speed up testing (#4239)

* Speed up testing

* make notification run in the background, this recudes the test_readme
time from 18s to 0.10s
* reduce time for zone reload

* TestServeDNSConcurrent remove entirely. This took a whopping 58s for
  ... ? A few minutes staring didn't reveal wth it is actually testing.
  Making values smaller revealed race conditions in the tests. Remove
  entirely.

* Move many interval values to variables so we can reset them to short
  values for the tests.

* test_large_axfr: make the zone smaller. The number used 64K has no
  rational, make it 64/10 to speed up.
* TestProxyThreeWay: use client with shorter timeout

A few random tidbits in other tests.

Total time saved: 177s (almost 3m) - which makes it worthwhile again to
run the test locally:

this branch:

~~~
ok  	github.com/coredns/coredns/test	10.437s
cd plugin; time go t ./...
5,51s user 7,51s system 11,15s elapsed 744%CPU (
~~~

master:

~~~
ok  	github.com/coredns/coredns/test	35.252s
cd plugin; time go t ./...
157,64s user 15,39s system 50,05s elapsed 345%CPU ()
~~~
tests/ -25s
plugins/ -40s

This brings the total on 20s, and another 10s can be saved by fixing
dnstapio. Moving this to 5s would be even better, but 10s is also nice.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Also 0.01

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/forward/persistent.go

@@ -152,7 +152,10 @@ const (
 	defaultExpire  = 10 * time.Second
 	minDialTimeout = 1 * time.Second
 	maxDialTimeout = 30 * time.Second
+)
 
+// Make a var for minimizing this value in tests.
+var (
 	// Some resolves might take quite a while, usually (cached) responses are fast. Set to 2s to give us some time to retry a different upstream.
 	readTimeout = 2 * time.Second
 )