test: Workaround slow SRV lookups in flaking test

[arjan-bal]

This test was migrated from grpc.DialContext to grpc.NewClient in #7920. This caused the resolver to change from passthrough to dns. The test started failing on Mac and flaking on debian systems (more than 1/100 runs fail). The test uses a short timeout and expects an RPC to fail due to a bad TLS certificate. When the test fails, the RPC failure cause is DeadlineExceeded. The SRV lookups for localhost and service grpclb are causing the timeouts. SRV lookup are disabled by default.

grpc-go/internal/resolver/dns/dns_resolver.go

Line 48 in e5a4eb0

EnableSRVLookups = false

SRV lookups are enabled when grpclb is imported.

grpc-go/balancer/grpclb/grpclb.go

Lines 105 to 108 in e5a4eb0

	func init() {
	balancer.Register(newLBBuilder())
	dns.EnableSRVLookups = true
	}

There is another file in the test package that imports grpclb, causing SRV lookups to be enabled for this test.

grpc-go/test/channelz_test.go

Line 35 in e5a4eb0

_ "google.golang.org/grpc/balancer/grpclb"

RELEASE NOTES: None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.08%. Comparing base (e5a4eb0) to head (e830617).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7957      +/-   ##
==========================================
- Coverage   82.21%   82.08%   -0.14%     
==========================================
  Files         379      379              
  Lines       38261    38261              
==========================================
- Hits        31458    31405      -53     
- Misses       5514     5551      +37     
- Partials     1289     1305      +16     

see 25 files with indirect coverage changes

[dfawley]

Maybe a quick note that the deadline is being shortened because WaitForReady waits for context expiration before allowing the call to return?

[arjan-bal]

Added a comment.