[gdb/testsuite] Fix fail in gdb.dap/sources.exp

With test-case gdb.dap/sources.exp, I run into:
...
{"request_seq": 4, "type": "response", "command": "source", \
  "success": false, "message": "notStopped", \
  "seq": 11}FAIL: gdb.dap/sources.exp: get source success
...

The fail happens because the request races with the stopping at main as
requested by:
...
if {[dap_launch $testfile stop_at_main 1] == ""} {
...

Fix this by waiting for the stop, in the same way that is done in other
test-cases that use stop_at_main.

Tested on x86_64-linux.

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

PR testsuite/31374
https://sourceware.org/bugzilla/show_bug.cgi?id=31374