Add tests related to Email Destination

[qreshi]

Issue #, if available:

Description of changes:
Add tests related to Email Destination changes

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #258 into master will increase coverage by 1.99%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #258      +/-   ##
============================================
+ Coverage     77.31%   79.31%   +1.99%     
- Complexity      187      188       +1     
============================================
  Files           137      137              
  Lines          4466     4466              
  Branches        594      594              
============================================
+ Hits           3453     3542      +89     
+ Misses          701      601     -100     
- Partials        312      323      +11     

Impacted Files	Coverage Δ	Complexity Δ
...h/alerting/model/destination/email/EmailAccount.kt	95.55% <0.00%> (+1.11%)	0.00% <0.00%> (ø%)
...icsearch/alerting/model/destination/email/Email.kt	81.33% <0.00%> (+4.00%)	0.00% <0.00%> (ø%)
...ing/destination/client/DestinationEmailClient.java	77.50% <0.00%> (+5.00%)	8.00% <0.00%> (+1.00%)
...rch/alerting/model/destination/email/EmailGroup.kt	88.60% <0.00%> (+5.06%)	0.00% <0.00%> (ø%)
...icsearch/alerting/model/destination/Destination.kt	72.34% <0.00%> (+7.09%)	0.00% <0.00%> (ø%)
...rting/transport/TransportSearchEmailGroupAction.kt	57.14% <0.00%> (+28.57%)	0.00% <0.00%> (ø%)
...ing/transport/TransportSearchEmailAccountAction.kt	57.14% <0.00%> (+28.57%)	0.00% <0.00%> (ø%)
...ticsearch/alerting/model/AlertingConfigAccessor.kt	90.00% <0.00%> (+40.00%)	0.00% <0.00%> (ø%)
...ing/model/destination/DestinationContextFactory.kt	66.66% <0.00%> (+55.55%)	0.00% <0.00%> (ø%)
...alerting/resthandler/RestSearchEmailGroupAction.kt	92.30% <0.00%> (+73.07%)	0.00% <0.00%> (ø%)
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 48ba52a...ab455d0. Read the comment docs.