Hi! I had exploring XmlUnit and found out that some test cases fails in assertj and assertj3 modules.
Here is a test results: failed_tests_result.zip (generated by Intellij IDEA).

These tests fails because I run them on Windows machine and where is a difference between Linux and Windows line separators.
On Linux line separator is \n and on Windows is \r\n.

So I propose to change \n in regular expressions to \R for failed tests.
Example:

//before
thrown.expectAssertionErrorPattern("^\\nExpecting:\\n <.*" + Pattern.quote(TestResources.TEST_RESOURCE_DIR) + "invalidBook.xml>\\nto be valid but found following problems:\\n.*");

//after
thrown.expectAssertionErrorPattern("^\\RExpecting:\\R <.*" + Pattern.quote(TestResources.TEST_RESOURCE_DIR) + "invalidBook.xml>\\Rto be valid but found following problems:\\R.*");

\R - is a special option in regex world and have meaning "any linebreak in Unicode". It's a synonym to (?:\r\n?|\n|\x0B|\f|\x85). (Wiki source)

Also some tests fails because javax.xml.parsers.DocumentBuilder always parse \r\n as \n. But assertions validate against %n which is formatted to \r\n in Windows.
To fix this kind of tests I propose to change

//before
thrown.expectAssertionError("Expected text value 'x' but was '%nx '");

//after
thrown.expectAssertionError("Expected text value 'x' but was '\nx '");

I will create a PR ASAP.