absl:: and benchmark:: flags seem to fight each other

Having benchmark:: and gtest tests together seems to produce some odd issues where the benchmark:: flags seem to get eaten by absl. This might be some issue with our configuration of these libraries.

Currently we work around this by using absl to parse the benchmark_filter flag and pass that down manually.

Take a look and try to remove this workaround at some point.

Start by locating the test configuration and the workaround that uses absl to parse benchmark_filter and pass it manually. Investigate how benchmark:: and absl flags are initialized when benchmark and gtest tests run together. Done means the workaround is removed and benchmark flags continue to work correctly in the combined test setup.

Written by the indexing model from the issue text.