Provide a build tool for testing Ion Schemas

[popematt]

It would be useful for Ion Schema users to have a tool for performing automated testing of their schemas to make sure that they are syntactically valid and they work as expected.

The test case format could be derived from (or a subset of) the existing ion-schema-tests syntax. Minimally, it would need to allow importing the types/schemas to be tested, and then specifying some values that are valid and invalid for those types (similar to the type definition tests and schema definition tests in ion-schema-tests).

In addition, the test runner could validate schemas against ion-schema-schemas.

It would be nice if this tool could be toolchain-agnostic so it can be used in any toolchain (gradle/npm/make/etc.).

[popematt]

This could also be solved by adding an ion schema test command to Ion CLI. However, for it to be very useful, it probably depends on being able to access locations of Element in ion-rust.