For policy-specific tests, use the version before the policy was
introduced. Otherwise, use 3.5 where possible.
Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.
Since commit 37da6af17d (find_dependency: Always search dependencies,
2019-03-29, v3.15.0-rc1~301^2) we search for a package every time it is
encountered in the transitive dependency graph traversal, even if it has
been found before. This was done in case a new visit to a given package
requests different components than a previous visit. However, it also
causes a lot of repeat processing that is unnecessary in the common case
that all calls share the same set of components (often none).
Restore the logic to avoid repeating the search for an already-found
package. Handle differences in components by de-duplicating based on
the complete `find_package` call arguments.
Fixes: #23685
Remove highly specialized and totally positional argument handling in
find_dependency macro, and instead just pass arguments through to
find_package. This gives users access to the full suite of arguments
that find_package knows, and is backward compatible with the old
arguments.
Also, rewrite the unit tests for this, since the old tests are
exclusively focused on testing the old argument handling and are no
longer applicable, and add some success tests (the old tests did not
even set up the CMake state in a way that CMake had any hope of ever
finding the test package).
