Merge branch 'fix/update_failing_2pfracturetests' into 'master'

[test][2p][fracture] fix failing 2p fracture tests

Closes #863

See merge request !2042

(cherry picked from commit 37d2dea0)

1182551f [test][2pfracture][box] Strengthen newton criteria
4c893ccf [test][2pfracture][timesteps] Add fixed time steps for reproducibility in sensitive tests
503ebaf8 [test][2pfracture][mpfa][gravity] Fixed time steps for the mpfa gravity test.
bc0d1a11 [test][2pfracture][cleanup] Remove periodic check point
5742f5f6 [test][2pfracture] Write out reference timestepsize files if no file is given
a787f229 [test][2pfracture][cleanup] Remove some unused stuff
5185bcc5 [test][2pfracture][tpfa] Run tpfa tests with fixed time steps for reproducibility