Fix some optional section tests on postgres

[hmpf]

Closes #152.

test_get_optionalsection, test_isskipped:

• problem due to factory_boy not handling unique_together well

For SectionFactory: see if setting both position and template fixes it, or else use Factory.build + instance.save instead of Factory.create, as create saves twice.

test_optional_section_in_summary:

• PK for first question in section 0 was not always 1 on postgres
• the length of section 2 is 3, since it is an optional section

[codecov[bot]]

Codecov Report

Merging #153 into master will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   60.99%   61.01%   +0.01%     
==========================================
  Files         110      110              
  Lines        6715     6718       +3     
==========================================
+ Hits         4096     4099       +3     
  Misses       2619     2619              

Impacted Files	Coverage Δ
tests/dmpt/models/test_Section.py	100.00% <100.00%> (ø)
tests/test_settings.py	100.00% <100.00%> (ø)

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