Exclude DHC.ETS.Combined.Subsystems.Validation.Borefield from OpenModelica tests

[mwetter]

In OpenModelica, Buildings.DHC.ETS.Combined.Subsystems.Validation.Borefield fails as the borefield outlet temperature drops below freezing. This is likely a simulation error.
This occurred for example on the master in
https://app.travis-ci.com/github/lbl-srg/modelica-buildings/jobs/636610285
and on maint_12.x in
https://app.travis-ci.com/github/lbl-srg/modelica-buildings/jobs/636610371#L448

This issue will exclude them from the OpenModelica tests on travis.
Future work should identify what commit causes the issue, and what the root cause is.