Fix slice handling in Env.Libraries() (#826)

Author: jpbetz

cel/env.go

@@ -390,7 +390,7 @@ func (e *Env) HasLibrary(libName string) bool {
 
 // Libraries returns a list of SingletonLibrary that have been configured in the environment.
 func (e *Env) Libraries() []string {
-	libraries := make([]string, len(e.libraries))
+	libraries := make([]string, 0, len(e.libraries))
 	for libName := range e.libraries {
 		libraries = append(libraries, libName)
 	}

cel/env_test.go

@@ -238,9 +238,13 @@ func TestLibraries(t *testing.T) {
 			t.Errorf("Expected HasLibrary() to return true for '%s'", expected)
 		}
 		libMap := map[string]struct{}{}
-		for _, lib := range e.Libraries() {
+		libraries := e.Libraries()
+		for _, lib := range libraries {
 			libMap[lib] = struct{}{}
 		}
+		if len(libraries) != 2 {
+			t.Errorf("Expected HasLibrary() to contain exactly 2 libraries but got: %v", libraries)
+		}
 
 		if _, ok := libMap[expected]; !ok {
 			t.Errorf("Expected Libraries() to include '%s'", expected)