feat(schematron): allow variables in Schematron scenario

[galtm]

I don't think there is any good reason for Schematron scenarios not to support XSpec variables. Omitting variable from the schematron-scenario pattern was probably just an oversight on my part when I worked on #475. The rest of the code seems to handle variables in Schematron test just fine.

This PR updates the schema to allow variables in Schematron scenarios and adds some tests.

[galtm]

The continuous-integration/appveyor/pr testing timed out three times, during the end-to-end testing phase. I checked in the latest log that the bats test ended with EXIT_CODE=0 and the non-end-to-end tests had only failed: 0. Some of the end-to-end tests are supposed to have failures. I doubt that whichever end-to-end tests didn't get run would have told us something new that the other two testing environments wouldn't have caught. AppVeyor testing sometimes takes around 45-55 minutes but times out occasionally.

At least temporarily, I removed "continuous-integration/appveyor/pr " from the list of required checks in the master branch protection rule.

[galtm]

AppVeyor passed all tests for #1838 (in only 46 min 52 sec), which was after this PR was already merged in.