test/channelz: Don't try to dial non-dialable addresses

[twz123]

On some platforms, dialing 127.0.0.1:0 doesn't fail immediately but rather blocks until it times out. This leads to test failures because the leak detector identifies stray goroutines.

Even if the tests don't need the dial to succeed, create a random listener and use that as resolved address.

RELEASE NOTES: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.40%. Comparing base (319a0fa) to head (b6734d9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8782      +/-   ##
==========================================
+ Coverage   83.33%   83.40%   +0.07%     
==========================================
  Files         418      418              
  Lines       32910    32910              
==========================================
+ Hits        27424    27450      +26     
+ Misses       4088     4070      -18     
+ Partials     1398     1390       -8     

see 35 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[eshitachandwani]

Instead of failing unconditionally , would a better way be to create a net.Listener and and pass it's address like :

lis, err := net.Listen("tcp", "localhost:0")
if err != nil {
    t.Fatalf("Failed to listen: %v", err)
}
defer lis.Close()

And then pass the address as

	resolvedAddrs := []resolver.Address{{Addr: lis.Addr().String(), ServerName: "grpclb.server"}}

[twz123]

Instead of failing unconditionally , would a better way be to create a net.Listener and and pass it's address like :

lis, err := net.Listen("tcp", "localhost:0")
if err != nil {
    t.Fatalf("Failed to listen: %v", err)
}
defer lis.Close()

And then pass the address as

	resolvedAddrs := []resolver.Address{{Addr: lis.Addr().String(), ServerName: "grpclb.server"}}

Right. I've tried this, and, funnily enough, when I allow the tests to dial, I encounter leak detector errors due to #8655. However, if I merge #8666 into this PR, the leak detector errors disappear.

I'll mark this one as draft until #8655 has been sorted out.