dns: drop test depending on invalid URL "dns://::1/foo.bar.com"

[neild]

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+.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.25%. Comparing base (50c6321) to head (23b996c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8716      +/-   ##
==========================================
+ Coverage   83.21%   83.25%   +0.03%     
==========================================
  Files         419      419              
  Lines       32427    32427              
==========================================
+ Hits        26985    26997      +12     
+ Misses       4054     4050       -4     
+ Partials     1388     1380       -8     

see 19 files with indirect coverage changes

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[arjan-bal]

LGTM. If I understand correctly, the change in url.Parse means gRPC will also reject unbracketed colons in the hostname. I believe this is fine since gRPC should remain consistent with the standard library. Adding another reviewer for confirmation.