[clangd][Hover] Make tests hermetic by setting target triplet

Summary: Fixes https://bugs.llvm.org/show_bug.cgi?id=44696

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73613