interop: Wait for server to become ready in alts tests

[arjan-bal]

In #7970, the alts client was changed from using grpc.Dial with grpc.WithBlock to grpc.NewClient without WithBlock. The tests that run in g3 don't wait for the server to start before starting the client. If WithBlock is not used, the client enters TRANSIENT_FAILURE after getting a connection refused error in the first connection attempt. This PR adds grpc.WaitForReady while making the RPC to restore the previous behaviour.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.26%. Comparing base (7505bf2) to head (a11f532).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8141      +/-   ##
==========================================
- Coverage   82.32%   82.26%   -0.07%     
==========================================
  Files         388      389       +1     
  Lines       39016    39125     +109     
==========================================
+ Hits        32121    32187      +66     
- Misses       5574     5607      +33     
- Partials     1321     1331      +10     

see 31 files with indirect coverage changes

[purnesh42H]

lgtm