Extract nvidia-smi binary lookup logic from GpuDiscoverer

The execution of testLinuxGpuResourceDiscoverPluginConfig depends on a system property: RunLinuxGpuResourceDiscoverPluginConfigTest. This is most likely the test executor creates the required files during test runs , making the tests not executable locally.

• The logic of looking up nvidia binary should be separated from the GpuDiscoverer.
• The testcase should also be separated to 3 different testcases, as the comment says in the code.