Add upstream's tests

[Klaim]

Currently the only test implemented will just try to build the Readme code from v3.3.0

Add tests from upstream/tests/...
These tests require gtest (see https://github.com/build2-packaging/googletest we have a package but it lacks gmock support at the moment).

[Wmbat]

gtest is now packaged in cppget, we should explore this further !

[Klaim]

Going back to this:

• Having gtest packaged was our initial requirement, it's now done so we can proceed.
• splitting the package in two is the first thing to do to get the original tests running without adding gtest as dependency to the library (and some others as specified below).
• Then as a fist step we need to add the "simple basic tests", which are tests using this macro in the upstream CMake for tests; this also includes the examples.
  • CAN BE IGNORED: We will need to package the cereal serialization library as some of these basic tests use it
• CAN BE IGNORE Then we can add the library tests which are a bit more complicated, they generate both a library and an executable linking to it. Once this is done we already have most of the coverage.
• CAN BE IGNOREDThen there are plugin case tests which are a bit more complex and probably requires some testscript scripting so that the plugin binary can be found at runtime when running the tests. Also note that for loading the plugins, they download a copy of this cr one-header library which is currently not packaged, so...
  • ...we need to package cr: https://github.com/fungos/cr

Edit: updated following the discussion

I will ignore the benchmarks for now.

• A special attention need to be done for some of the options/macros because they seem to be important, for examlpe the presence or not of ENTT_HAS_HEADER_VERSION;
• There is something that looks like a basic ODR check in there that all the tests requires, not sure what this is about exactly but seems like it can be done easilly.

I believe that covers everything https://github.com/skypjack/entt/blob/v3.11.1/test/CMakeLists.txt , please tell me if I'm missing something @skypjack

[skypjack]

@Klaim I think we can ignore lib and cereal as well as the benchmark and a few others maybe?
These are extra tests for specific purposes. However, the main testsuite covers more or less everything already.
Also consider that all the ones above are enabled or disabled via cmake using a few options. It's easy to ignore them in case.

[Klaim]

@skyjpack Thanks for the info. Ok so we will focus on the normal tests, make them by default and just make a separate ticket for if someone wants to add more of the upstream tests.

We have a plan.