Move `explore` tests to v2 and v3 namespaces

Prior to this commit, the `explore` http tests were under the
`experimental` namespace.  This commit simply moves them
to v2 and creates a copy of them for v3.

As of this commit, the tests pass in both places; this is a bug.
I'm saving the bugfix for a second commit in order to make the
diff more readable.
Fix `nodes` REST API to use correct version of wrapped queries

Prior to this commit, when the `v2/nodes` endpoint was being used
to wrap facts/resources queries, it was inadvertently using
v3 of those.  This commit separates the code and tests out so that
they each have their own explicit codepath, and adds tests
that cover this issue (relating to `sourcefile` vs `file`, etc.,
on the wrapped `resources` query).
Merge pull request #716 from cprice404/bug/1.5.x/node-endpoint-spans-versions-of-wrapped-endpoints

Fix `nodes` REST API to use correct version of wrapped queries
Fix bug in paging test utility code

At some point when we changed the API for paging to use
`include-total` instead of `count?` for indicating that
we'd like for the response to include a count header,
we missed a spot in the test utility code that was still
expecting it to be `count?`, and thus we lost test
coverage for the count-related functionality.

This commit fixes that oversight and adds more strict
validation to the arguments to the test utility function,
to ensure that it is being used properly by all callers.
Merge pull request #718 from cprice404/bug/1.5.x/missed-api-change-in-paging-test-utils

Fix bug in paging test utility code
Fix the precondition on paging test utils
Merge pull request #719 from cprice404/bug/1.5.x/missed-api-change-in-paging-test-utils

Fix the precondition on paging test utils
Merge remote-tracking branch 'origin/1.5.x'