CppunitTest_xmlsecurity_signing: turn on clang-format for this file

This had manual consistent formatting. Recently it was broken, so bring
back consisency by using clang-format.

(And move the "if conversion fails" comment above
CPPUNIT_ASSERT_MESSAGE() to avoid the need for an over-indented
comment.)

Change-Id: Id6a9231c044d7282c84a21152ffdfdcb8af3690d
Reviewed-on: https://gerrit.libreoffice.org/63327Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

solenv/clang-format/blacklist

@@ -19459,7 +19459,6 @@ xmlsecurity/inc/xsecfwdllapi.h
 xmlsecurity/inc/xsecgpgdllapi.h
 xmlsecurity/inc/xsecxmlsecdllapi.h
 xmlsecurity/qa/certext/SanCertExt.cxx
-xmlsecurity/qa/unit/signing/signing.cxx
 xmlsecurity/qa/unit/xmlsecurity-dialogs-test.cxx
 xmlsecurity/source/component/certificatecontainer.cxx
 xmlsecurity/source/component/certificatecontainer.hxx