Add ModfStruct and FrexpStruct to GLSL precision tests

Add ModfStruct and FrexpStruct to GLSL precision tests

This commit adds scalar and vectorized versions of precision tests to
check the ModfStruct and FrexpStruct extended instructions from
GLSL.std.450.

In addition, Modf tests also gain vectorized versions, compared to
scalar-only versions available previously.

New tests:
dEQP-VK.glsl.builtin.precision*.modfstruct.*
dEQP-VK.glsl.builtin.precision*.modf.*
dEQP-VK.glsl.builtin.precision*.frexpstruct.*

Affected tests:
dEQP-VK.glsl.builtin.precision*.frem.*
dEQP-VK.glsl.builtin.precision*.comparison.*
dEQP-VK.glsl.builtin.precision*.modfstruct.*

Components: Vulkan
VK-GL-CTS issue: 2296

Change-Id: I29b902ffec0a93a8cb13c30a440da008cce850f2