Update to the Riemann solver unit tests

Some changes to the Riemann solver unit tests:

• Generate random configurations and print out the seed in case we crash in order to investigate.
• Run 100000 cases to test the symmetry. Should cover all bases!
• Check differences in absolute and relative values.

Implements the suggestion at the end of the discussion in #424 (closed).

Looks good! No objections against merging this.

Thanks!

merged

mentioned in commit 2643736b

mentioned in issue #424 (closed)