[libcxxabi][test][NFC] Turn off clang-format for demangler test-case array (#73503)

Adding test-cases to the `cases` array causes `git clang-format` to
split the strings of many of the existing test-cases, making them harder
to read/work with in most cases.

This patch disables `clang-format` for the `cases` array so it doesn't
catch anyone off-guard in the future.