Cover with tests some corner cases in GoPsiImplUtil

[ignatov]

https://teamcity.jetbrains.com/viewLog.html?buildId=664128&buildTypeId=IntellijIdeaPlugins_Go_Test&tab=coverage_idea

Coverage Summary for Class: GoPsiImplUtil (com.goide.psi.impl)

[dlsniper]

@ignatov I can't see that tab so I can't help out.