test: Throw on invalid custom form field and value

[iamareebjamal]

For #7074

[niranjan94]

Here is an overview of what got changed by this pull request:

Clones added
============
- tests/all/integration/api/attendee/test_attendee_api.py  14
         

See the complete overview on Codacy

[codecov]

Codecov Report

Merging #7082 into development will increase coverage by 0.02%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development    #7082      +/-   ##
===============================================
+ Coverage        61.95%   61.98%   +0.02%     
===============================================
  Files              262      262              
  Lines            12981    12981              
===============================================
+ Hits              8043     8046       +3     
+ Misses            4938     4935       -3     

Impacted Files	Coverage Δ
app/api/helpers/custom_forms.py	100.00% <0.00%> (+6.66%)	⬆️

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