[gdb/testsuite] Fix gdb.base/bt-on-error-and-warning.exp with check-read1

[gdb/testsuite] Fix gdb.base/bt-on-error-and-warning.exp with check-read1

When running test-case gdb.base/bt-on-error-and-warning.exp with check-read1,
I run into:
...
(gdb) maint internal-error foobar^M
src/gdb/maint.c:82: internal-error: foobar^M
A problem internal to GDB has been detectedFAIL: \
  gdb.base/bt-on-error-and-warning.exp: problem=internal-error, mode=on: \
  scan for backtrace (GDB internal error)
Resyncing due to internal error.
,^M
...

The corresponding gdb_test_multiple in the test-case contains:
...
           -early -re "^A problem internal to GDB has been detected,\r\n" {
               incr header_lines
               exp_continue
           }
...
but instead this one triggers in gdb_test_multiple:
...
        -re ".*A problem internal to GDB has been detected" {
            fail "$message (GDB internal error)"
            gdb_internal_error_resync
            set result -1
        }
...

Fix this by likewise shortening the regexp to before the comma.

Tested on x86_64-linux.