Add test coverage for pipeline providedBy

Adding an additional case for when a Task tries to depend on a Resource
from a previous Task that doesn't exist. Had previously covered this
only in the case where the first Task tries to express a `providedBy`,
but this is a special case b/c the first Task should never have any
`providedBy` constraints.

pkg/apis/pipeline/v1alpha1/pipeline_validation_test.go

@@ -35,10 +35,19 @@ func TestPipelineSpec_Validate_Error(t *testing.T) {
 				tb.PipelineTask("foo", "foo-task"),
 			)),
 		},
+		{
+			name: "providedby is on first task",
+			p: tb.Pipeline("pipeline", "namespace", tb.PipelineSpec(
+				tb.PipelineDeclaredResource("great-resource", v1alpha1.PipelineResourceTypeGit),
+				tb.PipelineTask("foo", "foo-task",
+					tb.PipelineTaskInputResource("the-resource", "great-resource", tb.ProvidedBy("bar"))),
+			)),
+		},
 		{
 			name: "providedby task doesnt exist",
 			p: tb.Pipeline("pipeline", "namespace", tb.PipelineSpec(
 				tb.PipelineDeclaredResource("great-resource", v1alpha1.PipelineResourceTypeGit),
+				tb.PipelineTask("baz", "baz-task"),
 				tb.PipelineTask("foo", "foo-task",
 					tb.PipelineTaskInputResource("the-resource", "great-resource", tb.ProvidedBy("bar"))),
 			)),