test: use CHECK instead of EXPECT where necessary
GetPageSize() and OverrunGuardedBuffer currently use non-fatal EXPECT_* macros because GoogleTest does not allow the fatal variants ASSERT_* in non-void returning functions (i.e., in this file, nowhere outside of the TEST itself).
The EXPECT_* macros continue execution upon failure, but we really don't want that (and static analysis apparently does not like it either). Since we cannot use GoogleTest's ASSERT_* here, use our own CHECK_* instead of EXPECT_* outside of the TEST. Hopefully, this will finally pacify static analysis.