add PathFollow3D unit test

[angad-k]

part of #43440
checks :

• Sampling with unit offset
• Sampling with offset
• Behavior on removal of point from curve
• Behavior when unit offset is below 0 and above 1
• Behavior when offset is below 0 and above the max value
  haven't added the tests for h_offset and v_offset. Will add when h_offset and v_offset for PathFollow3D dont produce intended effect with rotation turned off #46312 gets resolved.

[akien-mga]

Thanks!

[aaronfranke]

Why was this test merged in this state? It's not included in test_main.cpp, so the test won't run or even be compiled. Additionally, there are a ton of errors if you open this file in a C++ IDE.

The first line of the test should be Ref<Curve3D> curve = memnew(Curve3D); or const Curve3D *curve = memnew(Curve3D);. The last line of the screenshot is missing a parenthesis at the end.

#46277 also has these problems, it wasn't added to test_main.cpp and is not compilable.

[angad-k]

Oh no, I'm sorry. I'll fix these and add a PR in a day or two.

[raulsntos]

I've already fixed it in PR #51372 since I was adding a test