[cmake] [interception] Remove duplicate gtest from test COMPILE_DEPS

Fix the gtest dependency to be included in DEPS only, rather than
in COMPILE_DEPS + DEPS. The former variable is apparently used to
provide unconditional dependencies, while the latter are only used
for non-standalone builds. Since they are concatenated, specifying gtest
in both is redundant. Furthermore, including it in COMPILE_DEPS causes
build failure for standalone builds where 'gtest' target is not present.

Differential Revision: https://reviews.llvm.org/D38839

llvm-svn: 315605