Fix “Go → Test” to run tests for current file

The current file can either be a regular source file (“foo.go”) or the test file (“foo_test.go”). In both cases though, the test/non-test counterpart must exist for this command to work.

Previously we would run ‘go test «path»’ which fails since ‘go test’ expects a package. Run ‘go help packages’ for how a package is defined.
Fix links in error output when running untitled documents

We now use the default auto-linking of ‘«file»:«line»[:«column»]’ as it handles the case where we are executing a temporary file due to the source document being untitled.
Change key equivalents: Go → Test is now ⇧⌘R and Go → Build is ⌘B

Also rename “Go → Test” to “Go → Run Tests” and move it up below the “Go → Run” action.