(bug 43828) possibility to filter experimentals in selenium tests

- this adds the possibility to mark specific selenium test cases
as experimental
- these tests can then be filtered out by setting
EXPERIMENTAL=false in configuration.yml
- this changeset does not include the actual marking of tests;
this is done in a later changeset

Change-Id: I8378cac06a0407014a5080035290e9f9658b937c
Merge "(bug 43828) possibility to filter experimentals in selenium tests"