Ignore test-only dependency cycles

Ignore test-only dependency cycles

Make check-source and the dependency graph generation ignore
module-level dependencies where only tests in the source module depend
on the destination module.  This could possibly be more fine-grained (in
valid cases, the tests should only depend on a well-defined set of
low-level modules, not on any higher-level module), but for now this is
likely fine.

Change-Id: I9d218f632c6155ad6ba21a218bfc7ec940593b51