Add tests for check_mdattr

[philsmart]

I am unable to test the following rule:

<!--
        Section 2.3 line 176.

        Assertions not permitted in the context of an EntitiesDescriptor.
    -->
    <xsl:template match="md:EntitiesDescriptor/md:Extensions/mdattr:EntityAttributes/saml:Assertion">
        <xsl:call-template name="error">
            <xsl:with-param name="m">Assertion may not appear in the EntityAttributes for an EntitiesDescriptor</xsl:with-param>
        </xsl:call-template>
    </xsl:template>

As the md-validator does not accept EntitiesDescriptor.

[iay]

That seems like an inevitable limitation of the way the testbed (currently) works. If you want to stash a note for later, feel free to include a README.md indicating anything like this that we don't have anywhere else to stash.