All_compare test: Expect "true" on single process

[hirschsn]

The all_compare test checks if all_compare correctly returns false if supplied with different values. This, however, only holds true if the test is called with more than one processes.

[codecov]

Codecov Report

Merging #3677 into python will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           python   #3677   +/-   ##
======================================
- Coverage      87%     87%   -1%     
======================================
  Files         536     536           
  Lines       22756   22756           
======================================
- Hits        20012   20010    -2     
- Misses       2744    2746    +2     

Impacted Files	Coverage Δ
src/utils/tests/all_compare_test.cpp	100% <100%> (ø)
src/core/particle_data.cpp	96% <0%> (-1%)	⬇️

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[fweik]

Thanks!