gdb/testsuite: work around empty substring bug in expect

gdb/testsuite: work around empty substring bug in expect

There is a bug in expect, see:

  https://sourceforge.net/p/expect/patches/26/

which causes empty substring matches from a regexp to instead return
the complete input buffer.  To reproduce this bug, try this command:

  expect -c 'spawn sh -c "echo -n -e \"abc\""; \
            expect -re "(a?)(a)(bc)"; \
	    puts "\n"; \
	    for { set i 1 } { $i < 4 } { incr i } { \
	      puts -nonewline "($i): \""; \
	      puts -nonewline $expect_out($i,string); \
	      puts "\"" \
	    }'

For a working expect the output looks like:

  spawn sh -c echo -n -e "abc"
  abc

  (1): ""
  (2): "a"
  (3): "bc"

But for a broken expect the output looks like:

  spawn sh -c echo -n -e "abc"
  abc

  (1): "abc"
  (2): "a"
  (3): "bc"

Notice that (1) is now returning the complete input buffer rather than
the empty string, this is wrong.

This is not the first time this bug has impacted GDB's testsuite,
this commit seems to be working around the same problem:

  commit e579b537353cd91cb8fac1eaeb69901d4936766f
  Date:   Sat Aug 16 20:32:37 2025 +0200

      [gdb/testsuite] Fix TUI tests on freebsd

I recently pushed this commit:

  commit 3825c972a636852600b47c242826313f4b9963b8
  Date:   Wed Jun 18 15:02:29 2025 +0100

      gdb: allow gdb.Color to work correctly with pagination

Which added gdb.python/py-color-pagination.exp.  Bug PR gdb/33321 was
then created as the test was failing on some hosts.  Turns out, this
is same expect bug.

The fix presented here is the same as for e579b537353cd91cb8, avoid
using optional regexp substrings at the start of a regexp, and instead
use two separate regexp patterns.  With this change in place, the test
now passes on all hosts.

There's no change in what is being tested after this commit.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33321

Approved-By: Tom de Vries <tdevries@suse.de>