Expose a test fixture helper class

[chapulina]

🎉 New feature

Summary

It's not always straightforward to write automated tests that leverage ign-gazebo from external projects. Internal to Gazebo, we have various helper classes in test/helpers that make it easier to write integration tests. The goal of this PR is to expose a public API that makes it easier to write tests similar to those.

The main class being added in this PR is the TestFixture. See the added example for how to use it in an external project together with GTest. Example usage from the classes's docs:

/// // Load a world with a fixture
/// ignition::gazebo::TestFixture fixture("path_to.sdf");
///
/// // Register callbacks, for example:
/// fixture.OnPostUpdate([&](const gazebo::UpdateInfo &,
///   const gazebo::EntityComponentManager &_ecm)
///   {
///     // Add expectations here
///   }
///
/// // Run the server
/// fixture.Server()->Run(true, 1000, false);

Besides test/helpers, a lot of the inspiration here is coming from @arjo129 's work on a separate project.

Differences from test/helpers

While working on the public API I made some improvements over what we are currently using internally:

• Added new Server::AddSystem APIs that allow us to load a plugin created on the fly, instead of needing to load a shared library. (i.e. no more need for libMockPlugin.so, see the HelperSystem instead)
• The TestFixture, unlike test/helpers/Relay, instantiates a server automatically. This saves even more boilerplate in test code.

Test it

Run the new standalone example. I made an effort to use classes like World and Model on the example test in order to make it as approachable as possible. (i.e. no Each calls)

TODO

TODO before merging:

• Add tests for Server::AddSystem
• Add tests for TestFixture
• I'd like to add a 2nd example test that is more realistic, sending commands and reading sensor data
• General cleanup (docs, style, etc)

TODO in the future:

• Tying into the simulation update loop is very powerful, but it can be inconvenient for doing one-time setup operations, such as getting a model's entity ID. Done in Add a way to perform startup checks using TestFixture. #930

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

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[arjo129]

Regarding the TODO I've opened #930. This will make it easier to perform tests at startup. Most of my comments are minor nitpicks.

[chapulina]

This PR is ready for review. I recommend starting by reviewing #935 and #936 first, which are subsets of this PR and will make it shorter.

Once this is approved targeted at Fortress, I'm planning to retarget to Citadel, update as needed, merge, and forward-port up to Fortress.

All addressed

[chapulina]

@mjcarroll , I merged from main and cleaned it up. It's ready for review 👍

[caguero]

It looks good to me. The only comment I have is that the example requires CMake 3.11 and Ubuntu Bionic ships with 3.10. We claim that Fortress is supported in Ubuntu Bionic. I think this is OK as it's one example and there are other alternative ways to install CMake.

[chapulina]

The PR was approved at 9963485 when targeting main (v6). I rebased onto ign-gazebo3 and updated the API to work there on a05623d.

I'm going to merge into ign-gazebo3 when CI comes back happy.

If anyone wants to use this with main before it's forward-ported, checkout 9963485.

[codecov]

Codecov Report

Merging #926 (a05623d) into ign-gazebo3 (ad27b27) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-gazebo3     #926      +/-   ##
===============================================
+ Coverage        77.80%   77.90%   +0.10%     
===============================================
  Files              220      221       +1     
  Lines            12579    12639      +60     
===============================================
+ Hits              9787     9847      +60     
  Misses            2792     2792              

Impacted Files	Coverage Δ
include/ignition/gazebo/Util.hh	100.00% <100.00%> (ø)
src/TestFixture.cc	100.00% <100.00%> (ø)

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