TestDistro: return dummy values from PackageSets related methods

Return dummy values from the following methods:
 - PackageSets
 - PayloadPackageSets
 - PackageSetsChains

Use package set names commonly used by recent distro definitions.
Package sets are based on values used by rpmmd mock implementation.

Adjust two Weldr API unit test check for the dummy values. Without
this fix, these unit tests would start failing after the move to
`rpmmd.DepsolvePackageSets()` in Weldr API compose handler.

internal/distro/test_distro/distro.go

@@ -12,6 +12,19 @@ import (
 	"github.com/osbuild/osbuild-composer/internal/rpmmd"
 )
 
+const (
+	// package set names
+
+	// build package set name
+	buildPkgsKey = "build"
+
+	// main/common os image package set name
+	osPkgsKey = "packages"
+
+	// blueprint package set name
+	blueprintPkgsKey = "blueprint"
+)
+
 type TestDistro struct {
 	name             string
 	releasever       string
@@ -173,7 +186,25 @@ func (t *TestImageType) PartitionType() string {
 }
 
 func (t *TestImageType) PackageSets(bp blueprint.Blueprint) map[string]rpmmd.PackageSet {
-	return nil
+	return map[string]rpmmd.PackageSet{
+		buildPkgsKey: {
+			Include: []string{
+				"dep-package1",
+				"dep-package2",
+				"dep-package3",
+			},
+		},
+		blueprintPkgsKey: {
+			Include: bp.GetPackages(),
+		},
+		osPkgsKey: {
+			Include: []string{
+				"dep-package1",
+				"dep-package2",
+				"dep-package3",
+			},
+		},
+	}
 }
 
 func (t *TestImageType) BuildPipelines() []string {
@@ -185,11 +216,13 @@ func (t *TestImageType) PayloadPipelines() []string {
 }
 
 func (t *TestImageType) PayloadPackageSets() []string {
-	return []string{}
+	return []string{blueprintPkgsKey}
 }
 
 func (t *TestImageType) PackageSetsChains() map[string][]string {
-	return map[string][]string{}
+	return map[string][]string{
+		osPkgsKey: {osPkgsKey, blueprintPkgsKey},
+	}
 }
 
 func (t *TestImageType) Exports() []string {