Pull request: all: imp tests, docs

Merge in DNS/adguard-home from imp-tests to master

Squashed commit of the following:

commit 15e1bd4ac38e95aa7dce716679d9a6bea43c5964
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jan 29 19:41:03 2021 +0300

    scripts: imp docs

commit bc54ce4e703dd4b2956636e0bd554073c9aa12c6
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jan 29 19:39:26 2021 +0300

    all: imp tests, docs

HACKING.md

@@ -98,6 +98,9 @@ The rules are mostly sorted in the alphabetical order.
  *  Prefer constants to variables where possible.  Reduce global variables.  Use
     [constant errors] instead of `errors.New`.
 
+ *  Program code lines should not be longer than one hundred (**100**) columns.
+    For comments, see the text section below.
+
  *  Unused arguments in anonymous functions must be called `_`:
 
     ```go

scripts/README.md

@@ -108,6 +108,8 @@ Optional environment:
  *  `GO`: set an alternarive name for the Go compiler.
  *  `RACE`: set to `0` to not use the Go race detector.  The default value is
     `1`, use the race detector.
+ *  `TIMEOUT_FLAGS`: set timeout flags for tests.  The default value is
+    `--timeout 30s`.
  *  `VERBOSE`: verbosity level.  `1` shows every command that is run and every
     Go package that is processed.  `2` also shows subcommands.  The default
     value is `0`, don't be verbose.

scripts/make/go-test.sh

@@ -32,10 +32,12 @@ else
 	race_flags='--race'
 fi
 
-go="${GO:-go}"
-cover_flags='--coverprofile ./coverage.txt'
-count_flags='--count 1'
+readonly go="${GO:-go}"
+readonly timeout_flags="${TIMEOUT_FLAGS:---timeout 30s}"
+readonly cover_flags='--coverprofile ./coverage.txt'
+readonly count_flags='--count 1'
 
 # Don't use quotes with flag variables because we want an empty space if
 # those aren't set.
-"$go" test $race_flags $count_flags $cover_flags $x_flags $v_flags ./...
+"$go" test $count_flags $cover_flags $race_flags $timeout_flags\
+	$x_flags $v_flags ./...