patch 8.1.2087: cannot easily select one test function to execute

Problem:    Cannot easily select one test function to execute.
Solution:   Support the $TEST_FILTER environment variable. (Ozaki Kiichi,
            closes #2695)

src/testdir/summarize.vim

@@ -8,7 +8,7 @@ if 1
       let g:failed += a:match+0
     elseif a:type ==# 'skipped'
       let g:skipped += 1
-      call extend(g:skipped_output, ["\t".a:match])
+      call extend(g:skipped_output, ["\t" .. a:match])
     endif
   endfunc
 
@@ -19,6 +19,10 @@ if 1
   let g:failed_output = []
   let output = [""]
 
+  if $TEST_FILTER != ''
+    call extend(g:skipped_output, ["\tAll tests not matching $TEST_FILTER: '" .. $TEST_FILTER .. "'"])
+  endif
+
   try
     " This uses the :s command to just fetch and process the output of the
     " tests, it doesn't actually replace anything.