[clang-tools-extra] [clangd] Respect llvm_shlib_dir in tests

Add llvm_shlib_dir to variables used in clangd test suite, consistently
to how it is used in the test suites of clang, clang-tools-extra
and a few other components.  This is necessary to ensure that
the correct shared libraries are used when building clang standalone --
otherwise, use_clang() sets LD_LIBRARY_PATH to the directory containing
the earlier system installation of clang rather than the just-built
library.

Differential Revision: https://reviews.llvm.org/D135062