Tests for erosion

[xyl96]

Select suitable tests for erosion features

[EmiliaJarochowska]

@xyl96 asked me what further tests are needed, in addition to the ones already there. @jhidding do you have any recommendations off the top of your head or should we dig into it?

for @xyl96 one task remains for sure:

• change the regression tests so that you set the tolerance, see https://docs.julialang.org/en/v1/stdlib/Test/#Other-Test-Macros