templates: add test for HeaderFunctions

[thaJeztah]

Before:

go test -test.coverprofile -
PASS
coverage: 65.2% of statements
ok  	github.com/docker/cli/templates	0.607s

After:

go test -test.coverprofile -
PASS
coverage: 95.7% of statements
ok  	github.com/docker/cli/templates	0.259s

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.54%. Comparing base (3590f94) to head (12dcc6e).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5541      +/-   ##
==========================================
- Coverage   60.92%   59.54%   -1.39%     
==========================================
  Files         345      345              
  Lines       23491    29090    +5599     
==========================================
+ Hits        14313    17322    +3009     
- Misses       8205    10795    +2590     
  Partials      973      973