Pull request: 3845 hosts fatality

Merge in DNS/adguard-home from 3845-hosts-fatality to master

Updates #3845.

Squashed commit of the following:

commit 1447efcc4066e0226feaebde01fcc632cb7b7432
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Nov 17 17:14:35 2021 +0300

    home: imp readability

commit e934499072e983e1111b6c976eb93e1d6017981b
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Nov 17 13:35:10 2021 +0300

    aghnet: imp more

commit ed9995ee52bd9ec3fa130f3f56989619184a6669
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Nov 17 13:05:56 2021 +0300

    all: imp docs, code

commit 7b0718a1a4a58a4fd5f1ba24c33792b0610c334f
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Nov 16 20:32:24 2021 +0300

    all: reduce hosts container fatality

internal/aghtest/testfs.go

@@ -0,0 +1,46 @@
+package aghtest
+
+import "io/fs"
+
+// type check
+var _ fs.FS = &FS{}
+
+// FS is a mock fs.FS implementation to use in tests.
+type FS struct {
+	OnOpen func(name string) (fs.File, error)
+}
+
+// Open implements the fs.FS interface for *FS.
+func (fsys *FS) Open(name string) (fs.File, error) {
+	return fsys.OnOpen(name)
+}
+
+// type check
+var _ fs.StatFS = &StatFS{}
+
+// StatFS is a mock fs.StatFS implementation to use in tests.
+type StatFS struct {
+	// FS is embedded here to avoid implementing all it's methods.
+	FS
+	OnStat func(name string) (fs.FileInfo, error)
+}
+
+// Stat implements the fs.StatFS interface for *StatFS.
+func (fsys *StatFS) Stat(name string) (fs.FileInfo, error) {
+	return fsys.OnStat(name)
+}
+
+// type check
+var _ fs.GlobFS = &GlobFS{}
+
+// GlobFS is a mock fs.GlobFS implementation to use in tests.
+type GlobFS struct {
+	// FS is embedded here to avoid implementing all it's methods.
+	FS
+	OnGlob func(pattern string) ([]string, error)
+}
+
+// Glob implements the fs.GlobFS interface for *GlobFS.
+func (fsys *GlobFS) Glob(pattern string) ([]string, error) {
+	return fsys.OnGlob(pattern)
+}