resolver: fix CNAME -> TXT recursion

[cee-dub]

Fixes #115, #123

• Pass non-canceled context to recursive resolveCNAMEs
• Append correct RR in resolveCNAMEs
• Keep TestMain at top of resolver_test.go
• Add TestGoogleCNAME
• Rename CheckTXT -> checkTXT

Output Changes:

Before:

$ go run ./cmd/dnsr/main.go 355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no TXT

;; RESULTS:
355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no.	      3600	IN	CNAME	355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no.wordcore.no.
;; TRUE	355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no	TXT
;; Elapsed: 1.4088205s

$ go run ./cmd/dnsr/main.go translate.google.com A

;; RESULTS:
translate.google.com.	      3600	IN	CNAME	www3.l.google.com.
;; TRUE	translate.google.com	A
;; Elapsed: 86.525208ms

After:

$ go run ./cmd/dnsr/main.go 355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no TXT

;; RESULTS:
355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no.	      3600	IN	CNAME	355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no.wordcore.no.
355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no.wordcore.no.	      3600	IN	TXT	foobar
;; TRUE	355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no	TXT
;; Elapsed: 1.582778084s

❯ dig +short 355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no TXT
355702a0-beac-4944-ae69-73c70d68b5c3.miniwoffer.no.wordcore.no.
"foobar"

$ go run ./cmd/dnsr/main.go translate.google.com A

;; RESULTS:
translate.google.com.	      3600	IN	CNAME	www3.l.google.com.
www3.l.google.com.	      3600	IN	A	142.250.114.101
www3.l.google.com.	      3600	IN	A	142.250.114.100
www3.l.google.com.	      3600	IN	A	142.250.114.102
www3.l.google.com.	      3600	IN	A	142.250.114.138
www3.l.google.com.	      3600	IN	A	142.250.114.139
www3.l.google.com.	      3600	IN	A	142.250.114.113
;; TRUE	translate.google.com	A
;; Elapsed: 144.930083ms

$ dig +short translate.google.com
www3.l.google.com.
142.250.113.139
142.250.113.113
142.250.113.102
142.250.113.101
142.250.113.100
142.250.113.138