Add Test Server Wait Helper Function

[krapie]

What this PR does / why we need it:

Add test server wait helper function WaitForServerToStart() to resolve Connection Refused error in server-related tests.

Which issue(s) this PR fixes:

Fixes #784

Special notes for your reviewer:

To valify this, add time.Sleep(10 * time.Second) inside server's listenAndServe() goroutine to reproduce Connection Refused error and test it with and without WaitForServerToStart().

func (s *Server) listenAndServe() error {
	go func() {
		time.Sleep(5 * time.Second)
		logging.DefaultLogger().Infof(fmt.Sprintf("serving RPC on %d", s.conf.Port))
...

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (94d6e30) 50.71% compared to head (a270cf6) 50.71%.

❗ Current head a270cf6 differs from pull request most recent head 420488c. Consider uploading reports for the commit 420488c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #787   +/-   ##
=======================================
  Coverage   50.71%   50.71%           
=======================================
  Files          70       70           
  Lines       10213    10213           
=======================================
  Hits         5180     5180           
  Misses       4512     4512           
  Partials      521      521           

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[hackerwins]

Thanks for your contribution.

I adjusted error messages by following the below guide:
https://github.com/uber-go/guide/blob/master/style.md#error-wrapping