Add more unit tests for String `insert` and `join`. #37924

Job	Run time
📊 Static checks / Code style, file formatting, and docs	27s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 27s
🍎 macOS / Editor (target=editor, tests=yes)	5m 36s
🌐 Web / Template w/o threads (target=template_release, threads=no)	7m 31s
🍎 macOS / Template (target=template_release)	3m 59s
🤖 Android / Editor (target=editor)	5m 7s
🍏 iOS / Template (target=template_release)	1m 58s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 42s
🐧 Linux / Editor w/ Mono (target=editor)	6m 19s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 8s
🏁 Windows / Editor (target=editor, tests=yes)	10m 45s
🐧 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 29s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	8m 32s
🏁 Windows / Editor w/ clang-cl (target=editor, tests=yes, use_llvm=yes)	5m 32s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	7m 33s
🏁 Windows / Template (target=template_release)	10m 0s
🐧 Linux / Template w/ Mono (target=template_release)	5m 49s
🐧 Linux / Minimal template (target=template_release, everything disabled)	2m 30s
🪲 Godot CPP / Build and test Godot CPP	8m 11s
	2h 20m 35s

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