test/fileutil: Add t.Helper() to CopyFile()
test/fileutil: Add t.Helper() to CopyFile()

I discovered the missing t.Helper() by chance, so I decided to look
for other potential places where I could add the t.Helper(). Only one
that looked interesting after a quick `rg "^func .*\(t \*testing.T"`
was CopyFile, so I think all helpers are covered for now.

Change-Id: I4943e6ad3906c2ce49c65e2d54f3a6b990989b14

aristanetworks/goarista,aristanetworks/goarista