Improve Code Coverage for System.Security.Cryptography.Xml (66.3%)

[krwq]

Here are the main areas which need improvement:

• SignedXml.CheckSignature(X509Certificate2 certificate, bool verifySignatureOnly) - currently no coverage - sample code should be sufficient for now
• System.Security.Cryptography.Xml.XmlDecryptionTransform - https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/System.Security.Cryptography.Xml_XmlDecryptionTransform.htm
• System.Security.Cryptography.Xml.SymmetricKeyWrap - https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/System.Security.Cryptography.Xml_SymmetricKeyWrap.htm
• System.Security.Cryptography.Xml.KeyInfoClause - https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/System.Security.Cryptography.Xml_KeyInfoClause.htm
• System.Security.Cryptography.Xml.EncryptedXml - https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/System.Security.Cryptography.Xml_EncryptedXml.htm - only most common scenarios
• System.Security.Cryptography.Xml.EncryptionMethod - https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/System.Security.Cryptography.Xml_EncryptionMethod.htm
• System.Security.Cryptography.Xml.CanonicalXmlEntityReference - https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/System.Security.Cryptography.Xml_CanonicalXmlEntityReference.htm - at minimum one or two tests triggering use of this class

Guidelines:

• focus on writing tests which provide most value (rarely used scenarios can be targeted later)
• focus on public entry points

cc: @anthonylangsworth @tintoy @peterwurzinger

[StanislavUshakov]

The 2nd item (System.Security.Cryptography.Xml.XmlDecryptionTransform) is done in PR 17546 from 52% to 93% link to coverage report
I will continue writing tests.

[StanislavUshakov]

The 3rd point (System.Security.Cryptography.Xml.SymmetricKeyWrap) is done in PR 17599

[StanislavUshakov]

@krwq @bartonjs Hi, I'm working on tests for EncrypteXml, found like there's a small issue in RoundtripSample1 EncryptedXmlTest.cs

edata.KeyInfo.AddClause(new KeyInfoEncryptedKey(ekey));
edata.KeyInfo = new KeyInfo();

Looks like the KeyInfo ibject is being overwritten by the empty one.
I can fix it alongside with my added tests.

[krwq]

@StanislavUshakov thank you for spotting this! If that is a simple fix then you may fix it either together or separately. If it will resurface some bug in the product please do it separately.

[StanislavUshakov]

5th point is done in PR 17684
Overall coverage is 68.2% for System.Security.Cryptography.Xml
Also I've removed

edata.KeyInfo = new KeyInfo();

from RoundtripSample1