Blender Git Loki

Alembic: fix unit test on Windows

There are two issues solved in this commit:

- Our Windows buildbot has slightly different floating point errors than
the Linux one, which meant a larger delta was required for float
comparisons.
- The test performs an export to a temporary Alembic file and
subsequently imports it. Deleting the temporary file was impossible on
Windows because it was still in use. This is now resolved by first
loading the default blend file before deleting the Alembic file.