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>

test/file_xfr_test.go

@@ -12,9 +12,9 @@ import (
 )
 
 func TestLargeAXFR(t *testing.T) {
-	// Build a large zone in text format.  It contains 64K AAAA RRs.
+	// Build a zone in text format. It contains 6.4K AAAA RRs. (this number is rather random)
 	var sb strings.Builder
-	const numAAAAs = 65536
+	const numAAAAs = 6553
 	sb.WriteString("example.com. IN SOA . . 1 60 60 60 60\n")
 	sb.WriteString("example.com. IN NS ns.example.\n")
 	for i := 0; i < numAAAAs; i++ {