More OPA conformance (#77)

- object.union
- object.union_n
- treat negative integers as two separate tokens (Sub and Number)
  when seen in arithmetic expressions
- Ensure that fully query string is parsed
- Evaluate queries in a separate module instead of the last read module.
  This correctly handles queries of the form `x = data.test.y` where x is
  already a ref in `data.test`
- Handle queries producing multiple outputs in test infrastructure
- Add tests for engine
- Add tests locking down valid queries
- Update opa.passing

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/interpreter/cases/engine/tests.yaml

@@ -0,0 +1,44 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+cases:
+  - note: trailing whitespace in query
+    data: {}
+    modules: ["package test"]
+    query: "1 + 1 -2          "
+    want_result: 0
+
+  - note: trailing chars in query
+    data: {}
+    modules: ["package test"]
+    query: "[1]]"
+    error: expecting EOF
+
+  - note: trailing expressions in query
+    data: {}
+    modules: ["package test"]
+    query: "1 2"
+    error: expecting EOF
+
+  - note: multiple statements in query
+    data: {}
+    modules: ["package test"]
+    query: |
+      a = [1, 2, 3]
+      true
+      y = 1 + 1
+    want_result:
+      a: [1, 2, 3]
+      y:  2
+      
+  - note: comprehensions in query
+    data: {}
+    modules: ["package test"]
+    query: |
+      true
+      [1, 2, 3][_]
+    want_result:
+      many!:
+        - [true, 1]
+        - [true, 2]
+        - [true, 3]
+