gallium/tests: Fix warning calculating absdiff
With clang building tests:
../src/gallium/tests/trivial/compute.c:1215:29: warning: taking the absolute value of unsigned type 'unsigned int' has no effect [-Wabsolute-value] if (abs(((uint32_t *)x)[j] - ^ ../src/gallium/tests/trivial/compute.c:1215:29: note: remove the call to 'abs' since unsigned values cannot be negative if (abs(((uint32_t *)x)[j] -
Still not convinced gallium/tests/ provide value these days but I digress.