Disable bracketed paste mode in GDB tests

Disable bracketed paste mode in GDB tests

I have a patch to import GNU readline 8.1 into GDB.  However, when
running the tests, there were a number of failures due to "bracketed
paste mode".  This is a terminal feature that readline 8.1 enables by
default.

The simplest way to work around this was to always make a ".inputrc"
for GDB tests that will tell readline to disable brackted paste mode.

gdb/testsuite/ChangeLog
2021-01-23  Tom Tromey  <tom@tromey.com>

	* lib/gdb.exp (default_gdb_init): Set INPUTRC to a cached file.