Add tests for "orders import"

A couple crashes have resulted from invalid orders JSON files - this should
help catch any regressions with error handling.

See #1770, #1893, #1894

test/plugins/orders.lua

@@ -0,0 +1,80 @@
+TMP_FILE_NAME = 'tmp-test'
+TMP_FILE_PATH = ('dfhack-config/orders/%s.json'):format(TMP_FILE_NAME)
+
+function run_orders_import(file_content)
+    local f = io.open(TMP_FILE_PATH, 'w')
+    f:write(file_content)
+    f:close()
+
+    return dfhack.with_finalize(
+        function()
+            os.remove(TMP_FILE_PATH)
+        end,
+        function()
+            return dfhack.run_command_silent{'orders', 'import', TMP_FILE_NAME}
+        end
+    )
+end
+
+function check_import_success(file_content)
+    local output, result = run_orders_import(file_content)
+    expect.eq(result, CR_OK)
+end
+
+function check_import_fail(file_content, comment, prefix)
+    comment = comment or ''
+    prefix = prefix or 'error'
+    local output, result = run_orders_import(file_content)
+    expect.eq(result, CR_FAILURE)
+    expect.true_(output:lower():startswith(prefix), ('%s: output missing "%s"'):format(comment, prefix))
+end
+
+function test.import_empty()
+    check_import_success('[]')
+end
+
+function test.import_non_array()
+    check_import_fail('{}', 'object', 'invalid')
+    check_import_fail('null', 'null', 'invalid')
+    check_import_fail('2', 'number', 'invalid')
+end
+
+function test.import_invalid_syntax()
+    -- for https://github.com/DFHack/dfhack/pull/1770
+    check_import_fail('', 'empty')
+    check_import_fail(']', 'missing opening bracket')
+    check_import_fail([[
+        [
+            {
+                    "amount_left" : 0,
+                    "amount_total" : 0,
+                    "frequency" : "OneTime",
+                    "id" : 0,
+                    "is_active" : false,
+                    "is_validated" : true,
+                    "job" : "CustomReaction",
+                    "reaction" : "BRASS_MAKING"
+            }
+    ]], 'missing closing bracket')
+end
+
+function test.import_invalid_id()
+    -- for https://github.com/DFHack/dfhack/issues/1893
+    check_import_fail([[
+        [
+            {
+                "amount_left": 0,
+                "amount_total": 0,
+                "frequency": "OneTime",
+                "id": "",
+                "is_active": false,
+                "is_validated": false,
+                "item_category": [
+                    "finished_goods"
+                ],
+                "job": "EncrustWithGems",
+                "material": "INORGANIC:AMBER OPAL"
+            }
+        ]
+    ]], 'string id instead of int')
+end