wishlist: add support for a `:GoTestFile` command

[dnwe]

What did you do? (required. The issue will be closed when not provided.)

I wanted to run all the test functions in the current _test.go file in the active buffer

What did you expect to happen?

I expected to have an option similar to :GoTestFunc or :GoTest but which would be scoped to issue a go test github.com/example/pkg/current -run ^(TestA|TestB|TestC)$ with a list of all the test functions in the current file in the -run param

What happened instead?

I was only able to run either all the tests for the current package (:GoTest) or just the tests for a single function with the current _test.go file in the active buffer (:GoTestFunc)

Configuration (MUST fill this out):

• vim-go version: git fatih/vim-go revision fatih/vim-go@a61545f

• vimrc you used to reproduce (use a minimal vimrc with other plugins disabled; do not link to a 2,000 line vimrc):

• Vim version (first three lines from :version): NVIM v0.3.4

• Go version (go version): go 1.11

• Go environment (go env): N/A