[Bugfix][Tests] Fix invalid copy assignment test in bcrsimplicitbuildtest
[Bugfix][BCRSMatrix] Improve error message when trying to copy-construct an incomplete matrix
[Bugfix][Tests] Some small fixes and improvements for bcrsbuildtest

- don't try to copy an incomplete matrix when passing it to rowWiseBuild
- test copy assignment in addition to copy construction
- make sure preallocated matrix can hold all inserted data