Fix tests to work with new version of screen

Debian unstable currently has a pre-release version of screen 4.1.0.
This version changes the "stuff" command so that special characters get
parsed, breaking many of the tests.

Detect if the version of screen being used to run tests does this
parsing, and if so do additional escaping of characters so that tests
again work.

With this change, all tests pass with both old and new versions of
screen.

test.py

@@ -92,14 +92,35 @@ class VimInterface:
 class VimInterfaceScreen(VimInterface):
     def __init__(self, session):
         self.session = session
+        self.need_screen_escapes = 0
+        self.detect_parsing()
 
     def send(self, s):
+        if self.need_screen_escapes:
+            # escape characters that are special to some versions of screen
+            repl = lambda m: '\\' + m.group(0)
+            s = re.sub( r"[$^#\\']", repl, s )
+
+        # Escape single quotes in command to protect from shell
         s = s.replace("'", r"'\''")
         cmd = "screen -x %s -X stuff '%s'" % (self.session, s)
         if sys.version_info >= (3,0):
             cmd = cmd.encode("utf-8")
         os.system(cmd)
 
+    def detect_parsing(self):
+        # Clear the buffer
+        self.send("bggVGd")
+
+        # Send a string where the interpretation will depend on version of screen
+        string = "$TERM"
+        self.send("i" + string + ESC)
+        output = self.get_buffer_data()
+
+        # If the output doesn't match the input, need to do additional escaping
+        if output != string:
+            self.need_screen_escapes = 1
+
 class VimInterfaceWindows(VimInterface):
     BRACES = re.compile("([}{])")
     WIN_ESCAPES = ["+", "^", "%", "~", "[", "]", "<", ">", "(", ")"]