Asserting on syntax paths in analysis tests

[jackfirth]

Currently, analysis-test test cases can only make assertions about analyzer property values being basic literal datums like strings, numbers, and symbols. But it's very useful for analyzers to add cross-references between different syntax objects using syntax paths, and analysis tests should be able to make assertions about that. One way for this to work would be to support the string syntax path format mentioned in #653 as a literal datum in #lang resyntax/test.

Another might be to do something like the @within ... @inspect ... options, like an @assertReferences <codeblock> @within <codeblock> alternative to @assert <value>, which would assert that the tested property value is a syntax path referring to a part of the test program identified by <codeblock> @within <codeblock>. Like @within <codeblock> @inspect <codeblock>, the @within <codeblock> part should be optional and its purpose is to disambiguate when the expected code block occurs multiple times within the test program.

The latter approach seems better from a user perspective, because it's difficult to tell what a syntax path corresponds to in a given textual program. Syntax paths are much more useful when looking at raw S-expressions than arbitrary surface syntax code strings.