Context regression tests (issue 348760043 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Message:
I would appreciate a close review of these tests by at least one of the
long-time contributors or pro users.  Contexts are a central part of
LilyPond and if I've misjudged how any of these cases should work, I
don't want it to slip by.  Thanks.

If you have suggestions on a better approach to testing, I'm all ears.

James, please leave this in review until there is feedback.

Description:
Context regression tests
Note that context-find-parent.ly currently fails.

Please review this at https://codereview.appspot.com/348760043/

Affected files (+271, -0 lines):
  A input/regression/context-find-child.ly
  A input/regression/context-find-current.ly
  A input/regression/context-find-grandchild.ly
  A input/regression/context-find-grandchild-ambiguous.ly
  A input/regression/context-find-parent.ly
  A input/regression/context-find-score-alias.ly
  A input/regression/context-find-sibling.ly
  A input/regression/context-global-find.ly
  A input/regression/context-global-new.ly
  A input/regression/context-global-set.ly
  A input/regression/context-new-child-same-name.ly
  A input/regression/context-new-sibling-same-name.ly