vim-patch:8.1.0044: if a test function exists Vim this may go unnoticed

Problem:    If a test function exists Vim this may go unnoticed.
Solution:   Check for a test funtion quitting Vim.  Fix tests that did exit
            Vim.
8903676d3d

Restore test_assert.vim to run Test_zz_quit_detected().

src/nvim/testdir/runtest.vim

@@ -124,7 +124,10 @@ func RunTheTest(test)
     exe 'call ' . a:test
   else
     try
+      let s:test = a:test
+      au VimLeavePre * call EarlyExit(s:test)
       exe 'call ' . a:test
+      au! VimLeavePre
     catch /^\cskipped/
       call add(s:messages, '    Skipped')
       call add(s:skipped, 'SKIPPED ' . a:test . ': ' . substitute(v:exception, '^\S*\s\+', '',  ''))
@@ -178,6 +181,15 @@ func AfterTheTest()
   endif
 endfunc
 
+func EarlyExit(test)
+  " It's OK for the test we use to test the quit detection.
+  if a:test != 'Test_zz_quit_detected()'
+    call add(v:errors, 'Test caused Vim to exit: ' . a:test)
+  endif
+
+  call FinishTesting()
+endfunc
+
 " This function can be called by a test if it wants to abort testing.
 func FinishTesting()
   call AfterTheTest()

src/nvim/testdir/test_assert.vim

@@ -0,0 +1,7 @@
+" Test that the methods used for testing work.
+
+" Must be last.
+func Test_zz_quit_detected()
+  " Verify that if a test function ends Vim the test script detects this.
+  quit
+endfunc