It would help a lot to mention where to get the test suite
and which exact test you run, and also study the test suite and try to analyze what is probably happening.
I did some fast research.
Failure message is referring to GML Coverage format. By WCS 2.0 core standard WCS 2.0 servers are not required to support this format:
while GML constitutes the canonical format for the definition of WCS,
it is not required by this core that a concrete instance of a WCS
service implements the GML coverage format
The failing test probably starts from this place in the core2main.xml:
<ctl:with-param name="url" select="$url"/>
<ctl:with-param name="Capabilities" select="$response"/>
<ctl:with-param name="SupportsGML" select="$SupportsGML"/>
In the WCS 2.0 standard the test for requirement 12 is defined as:
A.1.12 Version number Test id: /conf/core/version-number Test Purpose:
Requirement /req/core/version-number: For all WCS request types, the
request version parameter shall have a fixed value of “2.0.1”.
method: For each request type, send valid requests to server under
test. Modulate version parameter:
Overall test passes if all
individual tests deliver the result expected.
I can't say why the test fails. Test should be testing version number and according to the message it fails because a non-mandatory support for GML coverages is missing. Perhaps the initial error happens already before when "SupportsGML" is tested. If test interprets that the answer is TRUE even it in definitely FALSE with MapServer then strange things could happen in the next tests.