[gdb/testsuite] Reset errcnt and warncnt in default_gdb_init

[gdb/testsuite] Reset errcnt and warncnt in default_gdb_init

Say we do:
...
$ make check RUNTESTFLAGS="gdb.dap/ada-nested.exp gdb.dap/pause.exp"
...
and add a perror at the end of pause.exp:
...
 dap_shutdown
+
+perror "foo"
...

We run into:
...
UNRESOLVED: gdb.dap/ada-nested.exp: compilation prog.adb
...

This happens because the perror increases the errcnt, which is not reset at
the end of the test-case, and consequently the first pass in the following
test-case is changed into an unresolved.

Version 1.6.3 of dejagnu contains a fix which produces an unresolved at the
end of the test-case, which does reset the errcnt, but this is with version
1.6.1.

Furthermore, we reset the errcnt in clean_restart, but the pass is produced
before, so that doesn't help either.

Fix this by resetting errcnt and warncnt in default_gdb_init.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR testsuite/31351
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31351