[xmlsec] make check for signature-retrievalmethod-rawx509crt fail

[Roumen Petrov]

  ... when builddir != sourcedir.

Problem: files signature-retrievalmethod-rawx509crt.* contains 
URI="tests/keys/...." and builddir don't contain subdir tests.

One solution is to run script from Makefile as follow:
check-dsig: $(TEST_APP)
    @BUILDDIR="`pwd`";\
    (cd $(top_srcdir) && $(SHELL) $(top_srcdir)/tests/testDSig.sh 
$(top_srcdir)/tests $${BUILDDIR}/$(TEST_APP) der pkcs8 )