Add tests for `AnimationPlayer` #31622

Job	Run time
📊 Static checks / Code style, file formatting, and docs	57s
🤖 Android / Template (target=template_release)	30m 8s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	25m 13s
🍎 macOS / Editor (target=editor, tests=yes)	28m 41s
🌐 Web / Template w/o threads (target=template_release, threads=no)	24m 30s
🍎 macOS / Template (target=template_release)	38m 39s
🍏 iOS / Template (target=template_release)	8m 54s
🐧 Linux / Editor w/ Mono (target=editor)	40m 17s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	44m 33s
🏁 Windows / Editor (target=editor, tests=yes)	47m 42s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	33m 55s
🏁 Windows / Template (target=template_release)	33m 49s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	30m 53s
🐧 Linux / Template w/ Mono (target=template_release)	26m 27s
🐧 Linux / Minimal template (target=template_release, everything disabled)	15m 10s
🪲 Godot CPP / Build and test Godot CPP	7m 40s
	7h 17m 28s

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