gdb/testsuite: add DAP tests for core-file state-tracking bugs

[czidev-amd]

Add two tests in gdb.dap/ covering bugs reported against rocgdb 16.3, where GDB DAP's state tracker fell out of sync with the inferior after a core file was loaded.

corefile-cli-preload.exp
Spawn gdb with a core file already attached on the command line
(either '-ex "core-file "' or '-c '), then send one DAP
'initialize' request. In rocgdb 16.3 the response came back with
success=false and message="notStopped" because the pre-loaded core
was not registered as a stopped target. The test asserts
success=true; on failure, a follow-up assertion fires to flag
whether the specific "notStopped" message was returned.

corefile-after-launch.exp
Launch a binary with stopAtBeginningOfMainSubprogram, wait for the
stopped event at main, then load a core file by sending the CLI
command through a DAP 'evaluate' request in the repl context. The
evaluate succeeds and the new core inferior reaches stopped state,
but in rocgdb 16.3 every follow-up DAP request (threads, evaluate,
stackTrace, ...) returned success=false with message="notStopped"
because the state tracker missed the CLI-driven inferior switch.
The test issues 'threads', 'evaluate("info threads")', and
'stackTrace' and asserts each one succeeds.

Both tests reuse the existing gdb.dap/corefile.c source by setting testfile/srcfile/binfile manually; standard_testfile would otherwise derive the source name from the .exp basename.

The bugs do not reproduce against current trunk (gdb 18.0.50), so the tests pass today and will catch regressions if either bug returns.

[czidev-amd]

Bug-fixing commit:
0a481bb — "gdb/dap: add support for opening core files" by Andrew Burgess,
2026-03-23.

[motokultivator]

I tested, reviewed and it looks good to me.

[lumachad]

Only some nits. Furthermore, this should be pursue upstream rather than rocgdb. Once pushed upstream, we will pick this up from the regular master syncs.