Skip CLOCK_UPTIME_RAW_APPROX since it seems less precise than advertised on macOS

* See https://travis-ci.org/ruby/ruby/jobs/525595997

spec/ruby/core/process/clock_getres_spec.rb

@@ -27,7 +27,7 @@ describe "Process.clock_getres" do
           # The clock should not be less accurate than reported (times should
           # not all be a multiple of the next precision up, assuming precisions
           # are multiples of ten.)
-          times.select { |t| t % (reported * 10) == 0  }.size.should_not == times.size
+          times.select { |t| t % (reported * 10) == 0 }.size.should_not == times.size
         end
       end
     end

spec/ruby/core/process/fixtures/clocks.rb

@@ -26,6 +26,13 @@ module ProcessSpecs
       }
     end
 
+    # These clocks in practice on macOS seem to be less precise than advertised by clock_getres
+    platform_is :darwin do
+      clocks = clocks.reject { |clock, value|
+        [:CLOCK_UPTIME_RAW_APPROX].include?(clock)
+      }
+    end
+
     # These clocks in practice on ARM on Linux do not seem to match their reported resolution.
     platform_is :armv7l, :aarch64 do
       clocks = clocks.reject { |clock, value|