investigate remaining failurues in buffer_linestring_geo unit test

[vissarion]

Tests are failing due to an error in buffer_linestring_geo unit tests.
After bisecting the first commit that breaks that test is 805ff65
Interestingly, the cmake tests are passing since that test is disactivated (should we enable it by default?).

edit: the above is fixed. What is remaining is:

I will investigate them later this year, after integrating some other enhancements (robust side, #1404, ...)

[barendgehrels]

Thanks I'll have a look