Stabilize a test which was crashing at end of month

[fcayre]

The test checks contract forecasts' behaviour, which depends on the date the test is run. The problem is that contract recurring next date field is not that predictable as its behaviour at end of month dates is shaky. We circumvent this problem by choosing a date slightly in the past and not close to the end of a month.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (987ab2a) 90.47% compared to head (7e136a2) 90.45%.

Files	Patch %	Lines
rental_fees/tests/test_rental_fees_computation.py	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             12.0     #229      +/-   ##
==========================================
- Coverage   90.47%   90.45%   -0.03%     
==========================================
  Files         254      254              
  Lines        8408     8411       +3     
  Branches     1083     1084       +1     
==========================================
+ Hits         7607     7608       +1     
- Misses        632      633       +1     
- Partials      169      170       +1     

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