[gdb/testsuite] Add -q to INTERNAL_GDBFLAGS

[gdb/testsuite] Add -q to INTERNAL_GDBFLAGS

Whenever we start gdb in the testsuite, we have the rather verbose:
...
$ gdb
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(gdb)
...

This makes gdb.log longer than necessary and harder to read.

We do need to test that the output is produced, but that should be limited to
one or a few test-cases.

Fix this by adding -q to INTERNAL_GDBFLAGS, such that we simply have:
...
$ gdb -q
(gdb)
...

Tested on x86_64-linux.