Fix TabBar initialization issue and add tests #39946

Job	Run time
📊 Static checks / Code style, file formatting, and docs	25s
🍎 macOS / Editor (target=editor, tests=yes)	3m 57s
🍎 macOS / Template (target=template_release, tests=yes)	3m 34s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 4s
🌐 Web / Template w/o threads (target=template_release, threads=no)	7m 4s
🐧 Linux / Editor w/ Mono (target=editor)	3m 10s
🍏 iOS / Template (target=template_release)	1m 39s
🐧 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)	37m 0s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	7m 38s
🤖 Android / Editor (target=editor)	5m 4s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	6m 53s
🏁 Windows / Editor (target=editor, tests=yes)	14m 52s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	2m 54s
🐧 Linux / Template w/ Mono (target=template_release, tests=yes)	4m 5s
🏁 Windows / Editor w/ clang-cl (target=editor, tests=yes, use_llvm=yes)	5m 47s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 28s
🏁 Windows / Template (target=template_release, tests=yes)	8m 56s
🐧 Linux / Minimal template (target=template_release, tests=yes, everything disabled)	2m 18s
🪲 Godot CPP / Build and test Godot CPP	8m 30s
	2h 13m 18s

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