Improving notebook tests

What does this MR do and why?

The tests use let(:project) which makes the test suite really slow.

Screenshots or screen recordings

FPROF=1 bin/rspec spec/features/projects/commits/multi_view_diff_spec.rb spec/lib/gitlab/diff/rendered/notebook/diff_file_spec.rb spec/presenters/blobs/notebook_presenter_spec.rb

Before:

Finished in 4 minutes 49.3 seconds (files took 13.29 seconds to load)

[TEST PROF INFO] Factories usage

 Total: 71
 Total top-level: 37
 Total time: 03:14.030 (out of 04:58.276)
 Total uniq factories: 3

   total   top-level     total time      time per call      top-level time               name

      34          34      194.0047s            5.7060s           194.0047s            project
      34           0        4.5331s            0.1333s             0.0000s          namespace
       3           3        0.0261s            0.0087s             0.0261s            license

After:

Finished in 1 minute 50.37 seconds (files took 16.25 seconds to load)
34 examples, 0 failures

[TEST PROF INFO] Factories usage

 Total: 10
 Total top-level: 7
 Total time: 00:17.406 (out of 02:00.267)
 Total uniq factories: 4

   total   top-level     total time      time per call      top-level time               name

       3           3        0.0275s            0.0092s             0.0275s            license
       3           3       16.9970s            5.6657s            16.9970s            project
       3           0        0.3009s            0.1003s             0.0000s          namespace
       1           1        0.3822s            0.3822s             0.3822s               use

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