refactor tests

examples/foreach.yaml

@@ -0,0 +1,11 @@
+steps:
+  - name: foreach
+    foreach: "[1, 2, 3]"
+    command: sh
+    output: "foreach-$1.txt"
+    script: |
+      echo "Hello, world! $1"
+  - name: merge
+    command: sh
+    script: |
+      cat foreach-*.txt

internal/dag/builder.go

@@ -82,6 +82,7 @@ var (
 	errStepCommandMustBeArrayOrString = errors.New(
 		"step command must be an array of strings or a string",
 	)
+	errInvalidParamValue    = errors.New("invalid parameter value")
 	errCallFunctionNotFound = errors.New(
 		"call must specify a functions that exists",
 	)