test: simplify/improve osinfo_for() helper

test/test_build.py

@@ -453,11 +453,15 @@ def test_iso_installs(image_type):
 
 
 def osinfo_for(it: ImageBuildResult, arch: str) -> str:
+    osinfo = "Media is bootable.\nMedia is an installer for OS '{os}'\n"
+    os_str = ""
     if it.container_ref == "quay.io/centos-bootc/centos-bootc:stream9":
-        return f"CentOS Stream 9 ({arch})"
+        os_str = f"CentOS Stream 9 ({arch})"
     if it.container_ref.startswith("quay.io/fedora/fedora-bootc:"):
         ver = it.container_ref.split(":", maxsplit=2)[1]
-        return f"Fedora Server {ver} ({arch})"
+        os_str = f"Fedora Server {ver} ({arch})"
+    if os_str:
+        return osinfo.format(os=os_str)
     raise ValueError(f"cannot find osinfo_template for '{it.container_ref}'")
 
 
@@ -473,7 +477,7 @@ def test_iso_os_detection(image_type):
         installer_iso_path,
     ], capture_output=True, text=True, check=True)
     osinfo_output = result.stdout
-    expected_output = f"Media is bootable.\nMedia is an installer for OS '{osinfo_for(image_type, arch)}'\n"
+    expected_output = osinfo_for(image_type, arch)
     assert osinfo_output == expected_output