patch 9.0.0137: debugger test may fail when $CWD is very long

Problem:    Debugger test may fail when $CWD is very long.
Solution:   Skip the test if the directory name is too long. (James McCoy,
            closes #10837)

src/testdir/test_debugger.vim

@@ -364,6 +364,8 @@ endfunc
 
 " Test for expression breakpoint set using ":breakadd expr <expr>"
 func Test_Debugger_breakadd_expr()
+  CheckCWD
+
   let lines =<< trim END
     let g:Xtest_var += 1
   END