[clangd] Disable predefined macros in tests. NFC

These aren't needed. With them the generated predefines buffer is 13KB.
For every TestTU, we must:
 - generate the buffer (3 times: parsing preamble, scanning preamble, main file)
 - parse the buffer (again 3 times)
 - serialize all the macros it defines in the PCH
 - compress the buffer itself to write it into the PCH
 - decompress it from the PCH

Avoiding this reduces unit test time by ~25%.

Differential Revision: https://reviews.llvm.org/D125172