improve test output handling

* Use stricter regular expression to correctly identify potential file
  references in test output. When `go test` outputs a test failure file
  location, the file is preceded with at least one tab. If there is
  leading space on the line, then it's a subtest that failed. Use that
  to more accurately disregard lines that will not refer to files.

* Preserve multi-line test failure output by similarly checking for at
  least one tab ahead of the output.

* Preserve output that seemingly *should* refer to a file, but for
  whatever does not, but don't make it a file reference for the purposes
  of quickfix and location lists.

* Preserve all lines of stack traces.

* Correctly recognize file locations in stack traces.

autoload/go/test.vim

@@ -253,28 +253,50 @@ endfunction
 
 function! s:parse_errors(lines) abort
   let errors = []
+  let paniced = 0 " signals whether all remaining lines should be included in errors.
 
   " NOTE(arslan): once we get JSON output everything will be easier :)
   " https://github.com/golang/go/issues/2981
   for line in a:lines
-    let fatalerrors = matchlist(line, '^\(fatal error:.*\)$')
-    let tokens = matchlist(line, '^\s*\(.\{-}\.go\):\(\d\+\):\s*\(.*\)')
-
+    let fatalerrors = matchlist(line, '^\(\(fatal error\|panic\):.*\)$')
     if !empty(fatalerrors)
-      call add(errors, {"text": fatalerrors[1]})
-    elseif !empty(tokens)
+      let paniced = 1
+      call add(errors, {"text": line})
+      continue
+    endif
+
+    let tokens = []
+    if paniced
+      let tokens = matchlist(line, '^\t\+\(.\{-}\.go\):\(\d\+\) \(+0x.*\)')
+    else
+      let tokens = matchlist(line, '^ *\t\+\(.\{-}\.go\):\(\d\+\):\s*\(.*\)')
+    endif
+
+    if !empty(tokens) " Check whether the line may refer to a file.
       " strip endlines of form ^M
       let out = substitute(tokens[3], '\r$', '', '')
+      let file = fnamemodify(tokens[1], ':p')
+
+      " Preserve the line when the filename is not readable. This is an
+      " unusual case, but possible; any test that produces lines that match
+      " the pattern used in the matchlist assigned to tokens is a potential
+      " source of this condition. For instance, github.com/golang/mock/gomock
+      " will sometimes produce lines that satisfy this condition.
+      if !filereadable(file)
+        call add(errors, {"text": line})
+        continue
+      endif
 
       call add(errors, {
-            \ "filename" : fnamemodify(tokens[1], ':p'),
+            \ "filename" : file,
             \ "lnum"     : tokens[2],
             \ "text"     : out,
             \ })
+    elseif paniced
+      call add(errors, {"text": line})
     elseif !empty(errors)
-      " Preserve indented lines.
-      " This comes up especially with multi-line test output.
-      if match(line, '^\s') >= 0
+      " Preserve indented lines. This comes up especially with multi-line test output.
+      if match(line, '^ *\t\+') >= 0
         call add(errors, {"text": line})
       endif
     endif