Using fixtures in all qc system tests

[skymandr]

Reusing fixtures in all tests.

I'm a bit uncertain about the page_no_analysis fixture, since it is only used once, but I added it to make the code more uniform, in the hope that it makes it clearer over all.

[codecov-io]

Codecov Report

Merging #396 into master will increase coverage by 0.04%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #396      +/-   ##
==========================================
+ Coverage    41.2%   41.24%   +0.04%     
==========================================
  Files         207      207              
  Lines       16999    16999              
  Branches     2954     2954              
==========================================
+ Hits         7004     7011       +7     
+ Misses       9576     9553      -23     
- Partials      419      435      +16

Impacted Files	Coverage Δ
scanomatic/data_processing/phenotyper.py	27.51% <0%> (ø)	:arrow_up:
scanomatic/io/source.py	18.08% <0%> (ø)	:arrow_up:
scanomatic/ui_server/tools_api.py	8.77% <0%> (ø)	:arrow_up:
scanomatic/ui_server/data_api.py	9.65% <0%> (ø)	:arrow_up:
scanomatic/data_processing/phases/features.py	17.23% <0%> (ø)	:arrow_up:
scanomatic/ui_server/general.py	23.48% <0%> (ø)	:arrow_up:
scanomatic/qc/phenotype_results.py	0% <0%> (ø)	:arrow_up:
scanomatic/image_analysis/grid.py	76.69% <0%> (+0.75%)	:arrow_up:
scanomatic/data_processing/phases/segmentation.py	74.37% <0%> (+1.39%)	:arrow_up:

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