[gdb/testsuite] Fix gdb.ada/tagged-lookup.exp with gcc <= 12

With gcc 13, test-case gdb.ada/tagged-lookup.exp passes for me, but with gcc
12, I get:
...
(gdb) set debug symtab-create 1^M
(gdb) print *the_local_var^M
  ...
$1 = (n => 2)^M
(gdb) FAIL: gdb.ada/tagged-lookup.exp: only one CU expanded
...

The problem is that this fails:
...
    -re -wrap ".* = \\\(n => $decimal\\\)" {
	if {$found_pck + $found_pck2 == 1} {
	    pass $gdb_test_name
	} else {
	    fail $gdb_test_name
	}
...
because $found_pck == 0 and $found_pck2 == 0.

Indeed, with gcc 13 we have:
...
$ grep "start_subfile: name = .*/tagged-lookup/" gdb.log | sed 's%.*/%%'
b~foo.adb
b~foo.adb
b~foo.adb
b~foo.ads
pck2.adb
pck2.adb
pck2.ads
pck2.adb
pck2.ads
...
and with gcc 12:
...
$ grep "start_subfile: name = .*/tagged-lookup/" gdb.log | sed 's%.*/%%'
b~foo.adb
b~foo.adb
b~foo.adb
b~foo.ads
...

Fix this by checking for "$found_pck + $found_pck2 <= 1" instead.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR testsuite/31514
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31514