investigate remaining failurues in buffer_linestring_geo unit test

Tests are failing due to an error in buffer_linestring_geo unit tests.
After bisecting the first commit that breaks that test is https://github.com/boostorg/geometry/commit/805ff654e869030c4ba70f37fad6c3825f59c077
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, https://github.com/boostorg/geometry/pull/1404, ...)