dns: drop test depending on invalid URL "dns://::1/foo.bar.com" (#8716)

dns: drop test depending on invalid URL "dns://::1/foo.bar.com" (#8716)

Go 1.26's url.Parse will reject invalid URLs containing unbracketed
colons in the hostname.

For example, Go 1.25 and earlier are willing to parse the URLs
"https://localhost:80:443" (hostname:"localhost:80", port:443)
and "https://::1" (hostname:":", port:1).

The test TestCustomAuthority contains a case which depends on
url.Parse("dns://::1/foo.bar.com") succeeding. In Go 1.26, this
case will fail.

Drop the test as not exercising a useful path: This URL is invalid
and earlier Go versions being willing to parse it was a bug.
The correct URL is "dns://[::1]/foo.bar.com" (which is also
exercised by TestCustomAuthority).

RELEASE NOTES:
* client: Reject target URLs containing unbracketed colons in the
hostname in Go version 1.26+.