(#14652) Add check for self-dependency cycles

With some changes that were made to how the graph is processed, puppet
2.7.0 lost the ability to detect dependency cycles where a resource
requires itself. Instead of erroring, the resource ended up being
silently ignored. This restores the behavior that existed in 2.6 where
this was considered a cyclic dependency.

The erroneous code had considered any single vertex, strongly connected
component to not be a cycle. This was done because by definition any
single vertex is a strongly connected component, no matter what edges
are involved. By always excluding those vertices, the case of a
self-referencing vertex was not handled. This adds a check for a single
vertex SCC that is adjacent to itself in order to find dependency cycles
of a single resource.

Paired-with: Josh Cooper <josh@puppetlabs.com>
(#14652) Ensure SimpleGraph finds self-loop cycles
(Maint) Remove unneeded checks for no error raised

There is no need to check for no error being raised because the test
would fail if any error is raised.