Skip fconvert16 if float16 not supported

The SPIR-V instruction test for float16 wasn't checking for the float16
capability correctly so didn't work properly on implementations that
don't support it.

Components: Vulkan
VK-GL-CTS issue: 1499
Affects:
    dEQP-VK.spirv_assembly.instruction.compute.opspecconstantop.fconvert16

Change-Id: Id8dfc596acda41da670b2d6c08bde9920df99268

external/vulkancts/modules/vulkan/spirv_assembly/vktSpvAsmInstructionTests.cpp

@@ -2791,6 +2791,8 @@ tcu::TestCaseGroup* createSpecConstantGroup (tcu::TestContext& testCtx)
 		// Special SPIR-V code for FConvert-case for 16-bit floats
 		if (strcmp(cases[caseNdx].caseName, "fconvert16") == 0)
 		{
+			spec.extensions.push_back("VK_KHR_shader_float16_int8");
+			spec.requestedVulkanFeatures.extFloat16Int8 = EXTFLOAT16INT8FEATURES_FLOAT16;
 			specializations["CAPABILITIES"]			= "OpCapability Float16\n";							// Adds 16-bit float capability
 			specializations["OPTYPE_DEFINITIONS"]	= "%f16 = OpTypeFloat 16\n";						// Adds 16-bit float type
 			specializations["TYPE_CONVERT"]			= "%sc_final32 = OpConvertFToS %i32 %sc_final\n";	// Converts 16-bit float to 32-bit integer