cmd: run tests only if the test file exists

Closes #859

autoload/go/cmd.vim

@@ -188,6 +188,18 @@ endfunction
 function! go#cmd#Test(bang, compile, ...)
     let args = ["test"]
 
+    " only run if the file exists
+    let test_file = expand('%')
+    if test_file =~# '^\f\+\.go$'
+        let l:root = split(test_file, ".go$")[0]
+        let l:alt_file = l:root . '_test.go'
+
+        if !filereadable(alt_file) && !bufexists(alt_file)
+            call go#util#EchoError("couldn't find ".alt_file)
+            return
+        endif
+    endif
+
     " don't run the test, only compile it. Useful to capture and fix errors or
     " to create a test binary.
     if a:compile