Add test for email_case_deletion(). Refs #878

[atodorov]

Cherry-pick only the tests from #878 since the rest has already been introduced into master.

[codecov-io]

Codecov Report

Merging #969 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #969      +/-   ##
==========================================
+ Coverage   73.15%   73.34%   +0.18%     
==========================================
  Files         112      112              
  Lines        4764     4764              
  Branches      609      609              
==========================================
+ Hits         3485     3494       +9     
+ Misses       1057     1044      -13     
- Partials      222      226       +4

Impacted Files	Coverage Δ
tcms/core/history.py	90.69% <0%> (ø)	⬆️
tcms/signals.py	76.19% <0%> (+6.34%)	⬆️
tcms/testcases/helpers/email.py	82.85% <0%> (+14.28%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 3316

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 74.038%

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Totals
Change from base Build 3313:	0.2%
Covered Lines:	3719
Relevant Lines:	4764

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💛 - Coveralls