Touch watchdog more often for precision tests.

Have a test that takes ~2.5 minutes here using ARM
emulation:

01-15 17:58:54.903  3909  3926 I dEQP    : Test case 'dEQP-VK.glsl.builtin.precision.reflect.mediump_compute.vec4'..
01-15 18:01:28.631  3909  3926 I dEQP    :   Pass (Pass)

Since this is still below the overall 300 seconds limit
for a test I thought just increasing the watchdog touch
frequency would be okay.

We probably strictly need about 1000, but for leaving
some time before/after this for the driver to do stuff
(like compiling a shader), I took something lower.

Affected tests:
dEQP-VK.glsl.builtin.precision.*

Component: Vulkan

VK-GL-CTS issue: 1560

Change-Id: I450ddac39635c857ee1966d86ae16085082d242b
diff --git a/external/vulkancts/modules/vulkan/shaderexecutor/vktShaderBuiltinPrecisionTests.cpp b/external/vulkancts/modules/vulkan/shaderexecutor/vktShaderBuiltinPrecisionTests.cpp
index a5c09c7..717da3b 100644
--- a/external/vulkancts/modules/vulkan/shaderexecutor/vktShaderBuiltinPrecisionTests.cpp
+++ b/external/vulkancts/modules/vulkan/shaderexecutor/vktShaderBuiltinPrecisionTests.cpp
@@ -69,7 +69,7 @@
 	// platforms where toggling floating-point rounding mode is slow (emulated arm on x86).
 	// As a workaround watchdog is kept happy by touching it periodically during reference
 	// interval computation.
-	TOUCH_WATCHDOG_VALUE_FREQUENCY	= 4096
+	TOUCH_WATCHDOG_VALUE_FREQUENCY	= 512
 };
 
 namespace vkt