TDML Runner not caching schema compiles - only caches schemas if compileAllTopLevel is true

The TDML runner is supposed to cache compiled schemas so that many tests using the same DFDL schema will compile it only once.

It currently only does this when compileAllTopLevel is true, but the default for this is false.

The cache should be modified to take the root element into account - i.e., compile all top levels need not be true. The cache should hit, avoiding recompilation, so long as the same root element is being used.