TestNullPointer: avoid unnecessary Settings creations

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Some of these tests showed up in the slowest valgrind ones.

Clang 22 8,765,375,304 -> 562,138,632

before

Benchmark 1: bin/testrunner TestNullPointer
  Time (mean ± σ):     780.7 ms ±   2.7 ms    [User: 737.3 ms, System: 43.2 ms]
  Range (min … max):   776.6 ms … 785.8 ms    10 runs

after

Benchmark 1: bin/testrunner TestNullPointer
  Time (mean ± σ):     152.5 ms ±   0.9 ms    [User: 115.5 ms, System: 36.9 ms]
  Range (min … max):   151.1 ms … 154.9 ms    19 runs

Unfortunately I could not run it in valgrind as it is currently broken on my distro.