tests/: spelling fixes

with some rewording in
 tests/qemu-iotests/298
 tests/qtest/fuzz/generic_fuzz.c
 tests/unit/test-throttle.c
as suggested by Eric.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>