tests: migrate ops test to bats

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

tests/ops/exec/undefined/non_concrete_referenced/main.cue

@@ -0,0 +1,20 @@
+package testing
+
+hello: "world"
+bar:   string
+
+#up: [
+	{
+		do:  "fetch-container"
+		ref: "alpine"
+	},
+	{
+		do:  "exec"
+		dir: "/"
+		args: ["sh", "-c", """
+		echo \(bar)
+		echo "This test SHOULD succeed, because this is never going to be executed, as \(bar) is not concrete"
+		exit 1
+		"""]
+	},
+]