Explicitly disable bracketed paste mode in test sessions

Since Debian 11 (bash 5.1) bracketed paste mode is turned on by default.
This leads to wrapping each command output into '\e[?2004l' and '\e[?2004h'
(disable bracketed paste, print to stdout, enable bracketed paste).

Since we trust any stdin input in our test suite, there is no reason to
leave this feature on and to manually process garbled output.

Links:
    http://cirw.in/blog/bracketed-paste
    syl20bnr/spacemacs#3035

tests/common.py

@@ -28,6 +28,7 @@ class BashSession:
 
     ENCODING = 'utf-8'
     STARTUP = [
+        "bind 'set enable-bracketed-paste off'",
         "bind 'set bell-style none'",
     ]
     ENV_UNSET = [