Refactor tests

Organize tests into subdirectories and use a new makefile that scans
.sby files and allows selecting tests by mode, engine, solver and/or
subdirectory. Automatically skips tests that use engines/solvers that
are not found in the PATH.

See `cd tests; make help` for a description of supported make targets.

tests/junit/validate_junit.py

@@ -0,0 +1,19 @@
+from xmlschema import XMLSchema, XMLSchemaValidationError
+import argparse
+
+def main():
+    parser = argparse.ArgumentParser(description="Validate JUnit output")
+    parser.add_argument('xml')
+    parser.add_argument('--xsd', default="JUnit.xsd")
+
+    args = parser.parse_args()
+
+    schema = XMLSchema(args.xsd)
+    try:
+        schema.validate(args.xml)
+    except XMLSchemaValidationError as e:
+        print(e)
+        exit(1)
+
+if __name__ == '__main__':
+    main()