gdb/testsuite: bump duration for the whole test in do_self_tests

gdb/testsuite: bump duration for the whole test in do_self_tests

When running gdb.gdb/python-helper.exp, I get some timeouts:

    continue
    Continuing.
    print 1

    FAIL: gdb.gdb/python-helper.exp: hit breakpoint in outer gdb (timeout)

At this time, GDB is actually processing the stop and reading in some
CUs.  selftest_setup does bump the timeout, but it's not for the whole
test.

Since debugging GDB with GDB is (unfortunately) a bit slow, bump the
timeout for the whole duration of the setup and body.  On my optimized
build, the command takes just a bit more than the current timeout of 10
seconds.  But it's much slower if running the test on an unoptimized
build, so I think it's necessary to bump the timeout for that in any
case.

Change-Id: I4d38285870e76c94f9d0bfdb60648a2e7f2cfa5d