Fix shader operator tests' precision expectation

Fix shader operator tests' precision expectation

es3fShaderOperatorTests.cpp takes the reported precision for lowp and
mediump (through glGetShaderPrecisionFormat) and tests that that many
bits are actually supported by the type.

glGetShaderPrecisionFormat is a legacy function that's not very
meaningful on modern hardware (see Vulkan issue #2931), and there is no
equivalent for its functionality in Vulkan.

This relevant tests are changed such that the test now verifies that _at
least_ that many bits are used in lowp and mediump operations, rather
than _exactly_.  This aligns with the semantics of RelaxedPrecision in
SPIR-V.

Affected tests:
dEQP-GLES3.functional.shaders.operator.*

Components: OpenGL ES
VK-GL-CTS issue: 3344

Change-Id: I88a38d6ee067de8313f25327578c6666c257dc8b