[clang][ExtractAPI][NFC] Fix sed delimiter in test

Fix path replacement in sed (properly this time) using lit
regex_replacement.

Differential Revision: https://reviews.llvm.org/D123526



Co-authored-by: Michele Scandale <michele.scandale@gmail.com>
Co-authored-by: Zixu Wang <9819235+zixu-w@users.noreply.github.com>