Test for issue 700 (Catch error messages from xsl:message terminate=yes)

[AirQuick]

Closes #700

This pull request adds a test for #700.

• The test is skipped until Saxon implements the required feature. Right now, the test assumes the implementation to happen on the next maintenance release of 9.9. If postponed, this pull request needs to be updated accordingly.
• Right now, the 2nd x:expect expects a string. But x:result?err?value will be a node. The x:expect in question will need to be updated using TVT (feat: support Text Value Templates (expand-text) #711) or another way.

[galtm]

In #749, the test started checking for the processing instruction mentioned in https://saxonica.plan.io/issues/4376#note-2 :

Note that Saxon inserts a processing instruction into the xsl:message document containing the error code and the location of the xsl:message instruction. This is convenient, but not strictly according to spec.

Saxon 11 does not insert that PI. The change appears to be intentional; notice the differences in the Details section between these pages of the Saxon documentation for versions 10 and 11:

https://saxonica.com/documentation10/index.html#!xsl-elements/message
https://saxonica.com/documentation11/index.html#!xsl-elements/message

I'm working on adding Saxon 11 testing to XSpec. Because this message treatment has changed in Saxon and because the XSLT spec does not require the PI anyway, I'm inclined to loosen the expectation in this test to stop looking for a PI. If you prefer a different resolution, please comment, either here or in an upcoming pull request that I create about Saxon 11 compatibility.