Add Layer Metadata test as living doc (#1019)

Layer Metadata is straightforward, adding a test to act as a living
document.

Signed-off-by: Aidan Delaney <adelaney21@bloomberg.net>

buildpack/layermetadata_test.go

@@ -0,0 +1,44 @@
+package buildpack_test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/sclevine/spec"
+	"github.com/sclevine/spec/report"
+
+	"github.com/buildpacks/lifecycle/buildpack"
+	h "github.com/buildpacks/lifecycle/testhelpers"
+)
+
+func TestLayerMetadata(t *testing.T) {
+	spec.Run(t, "unit-layermetadata", testLayerMetadata, spec.Report(report.Terminal{}))
+}
+
+func testLayerMetadata(t *testing.T, when spec.G, it spec.S) {
+	when("#LayerMetadata", func() {
+		var (
+			metadataFile *os.File
+		)
+		it.Before(func() {
+			var err error
+			metadataFile, err = os.CreateTemp("", "test")
+			h.AssertNil(t, err)
+		})
+		it.After(func() {
+			os.Remove(metadataFile.Name())
+		})
+		it("decodes file correctly", func() {
+			err := os.WriteFile(metadataFile.Name(), []byte("[types]\ncache = true"), 0400)
+			h.AssertNil(t, err)
+
+			var lmf buildpack.LayerMetadataFile
+			lmf, s, err := buildpack.DecodeLayerMetadataFile(metadataFile.Name(), "0.9")
+			h.AssertNil(t, err)
+			h.AssertEq(t, s, "")
+			h.AssertEq(t, lmf.Cache, true)
+			h.AssertEq(t, lmf.Build, false)
+			h.AssertEq(t, lmf.Launch, false)
+		})
+	})
+}