[gdb/testsuite] Add check-readmore

Consider the gdb output:
...
27        return SYSCALL_CANCEL (nanosleep, requested_time, remaining);^M
(gdb) ^M
Thread 2 "run-attach-whil" stopped.^M
...

When trying to match the gdb prompt using gdb_test which uses '$gdb_prompt $',
it may pass or fail.

This sort of thing needs to be fixed (see commit b0e2f96b), but there's
currently no way to reliably find this type of FAILs.

We have check-read1, but that one actually make the test pass reliably.

We need something like the opposite of check-read1: something that makes
expect read a bit slower, or more exhaustively.

Add a new test target check-readmore that implements this.

There are two methods of implementing this in read1.c:
- the first method waits a bit before doing a read
- the second method does a read and then decides whether to
  return or to wait a bit and do another read, and so on.

The second method is potentially faster, has less risc of timeout and could
potentially detect more problems.  The first method has a simpler
implementation.

The second method is enabled by default.  The default waiting period is 10
miliseconds.

The first method can be enabled using:
...
$ export READMORE_METHOD=1
...
and the waiting period can be specified in miliseconds using:
...
$ export READMORE_SLEEP=9
...

Also a log file can be specified using:
...
$ export READMORE_LOG=$(pwd -P)/LOG
...

Tested on x86_64-linux.

Testing with check-readmore showed these regressions:
...
FAIL: gdb.base/bp-cmds-continue-ctrl-c.exp: run: stop with control-c (continue)
FAIL: gdb.base/bp-cmds-continue-ctrl-c.exp: attach: stop with control-c (continue)
...

I have not been able to find a problem in the test-case, and I think it's the
nature of both the test-case and readmore that makes it run longer.  Make
these pass by increasing the alarm timeout from 60 to 120 seconds.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27957