DoS_tests: fix signed/unsigned comparison warnings

test/DoS_tests.cpp: In member function ‘void DoS_tests::DoS_mapOrphans::test_method()’:
test/DoS_tests.cpp:200:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
test/DoS_tests.cpp:208:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
test/DoS_tests.cpp: In member function ‘void DoS_tests::DoS_checkSig::test_method()’:
test/DoS_tests.cpp:260:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
test/DoS_tests.cpp:267:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
test/DoS_tests.cpp:280:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
test/DoS_tests.cpp:307:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]