tests: Cleanup xfail tests

[jvesely]

Clarify xfail error in OutputPort test.
Skip rather than xfail tests that are not supported.

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.11):

diff -r docs-base/ModulatoryProjection.html docs-head/ModulatoryProjection.html
272c272
< <a class="reference internal" href="Port.html"><span class="doc">Port</span></a> are listed in the Port’s <a class="reference internal" href="Mechanism.html#id7" title="psyneulink.core.components.mechanisms.mechanism.Mechanism_Base.mod_afferents"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">mod_afferents</span></code></a> attribute.</p>
---
> <a class="reference internal" href="Port.html"><span class="doc">Port</span></a> are listed in the Port’s <a class="reference internal" href="ParameterPort.html#psyneulink.core.components.ports.parameterport.ParameterPort.mod_afferents" title="psyneulink.core.components.ports.parameterport.ParameterPort.mod_afferents"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">mod_afferents</span></code></a> attribute.</p>
diff -r docs-base/Port.html docs-head/Port.html
616c616
< <code class="xref any docutils literal notranslate"><span class="pre">PathWayProjections</span></code> (listed in its <a class="reference internal" href="Mechanism.html#id6" title="psyneulink.core.components.mechanisms.mechanism.Mechanism_Base.path_afferents"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">path_afferents</span></code></a> attribute) as the variable for its
---
> <code class="xref any docutils literal notranslate"><span class="pre">PathWayProjections</span></code> (listed in its <a class="reference internal" href="#psyneulink.core.components.ports.port.Port_Base.path_afferents" title="psyneulink.core.components.ports.port.Port_Base.path_afferents"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">path_afferents</span></code></a> attribute) as the variable for its
diff -r docs-base/TransferFunctions.html docs-head/TransferFunctions.html
2263c2263
< <p><a class="reference internal" href="#psyneulink.core.components.functions.transferfunctions.Exponential.bounds" title="psyneulink.core.components.functions.transferfunctions.Exponential.bounds"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">bounds</span></code></a> – specifies the lower and upper limits of the result;  if there are none, the attribute is set to
---
> <p><a class="reference internal" href="OptimizationFunctions.html#psyneulink.core.components.functions.optimizationfunctions.GradientOptimization.bounds" title="psyneulink.core.components.functions.optimizationfunctions.GradientOptimization.bounds"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">bounds</span></code></a> – specifies the lower and upper limits of the result;  if there are none, the attribute is set to

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