patch 8.2.0866: not enough tests for buffer writing

Problem:    Not enough tests for buffer writing.
Solution:   Add more tests. Use CheckRunVimInTerminal in more places.
            (Yegappan Lakshmanan, closes #6167)
diff --git a/src/testdir/test_signals.vim b/src/testdir/test_signals.vim
index 48e3a3e..135037a 100644
--- a/src/testdir/test_signals.vim
+++ b/src/testdir/test_signals.vim
@@ -80,6 +80,7 @@
 
 " Test signal INT. Handler sets got_int. It should be like typing CTRL-C.
 func Test_signal_INT()
+  CheckRunVimInTerminal
   if !HasSignal('INT')
     throw 'Skipped: INT signal not supported'
   endif
@@ -91,9 +92,6 @@
     throw 'Skipped: cannot test signal INT with valgrind'
   endif
 
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot run vim in terminal'
-  endif
   let buf = RunVimInTerminal('', {'rows': 6})
   let pid_vim = term_getjob(buf)->job_info().process
 
@@ -120,9 +118,7 @@
   if !HasSignal('TERM')
     throw 'Skipped: TERM signal not supported'
   endif
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot run vim in terminal'
-  endif
+  CheckRunVimInTerminal
   let cmd = GetVimCommand()
   if cmd =~ 'valgrind'
     throw 'Skipped: cannot test signal TERM with valgrind'