Fix form attr references in tests

[rpkilby]

No description provided.

[codecov-io]

Codecov Report

Merging #789 into develop will decrease coverage by 0.4%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #789      +/-   ##
===========================================
- Coverage    98.36%   97.95%   -0.41%     
===========================================
  Files           15       15              
  Lines         1220     1220              
===========================================
- Hits          1200     1195       -5     
- Misses          20       25       +5

Impacted Files	Coverage Δ
django_filters/rest_framework/backends.py	92.85% <ø> (ø)	⬆️
django_filters/filters.py	97.28% <0%> (-1.21%)	⬇️
django_filters/utils.py	98.21% <0%> (-0.9%)	⬇️

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