Avoid readline conflicts in pexpect

In some cases, sensitive to readline version and the user's
environment, gdb might emit escape codes while run under python's
pexpect (i.e. testing pretty printers).  This patch, suggested
by Jan, helps isolate the test from the user's environment.

Tested on RHEL 7 x86_64 with DTS 7 and EPEL, which is one
magic combination of components that triggers this bug.