test: Move to f-string formatting instead of % formatting.

BUG=none
TEST=unittests

Change-Id: Ib6465a16ea4f87e0070358d31462a7794ef8a674
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3277121
Tested-by: Sean McAllister <smcallis@google.com>
Commit-Queue: Sean McAllister <smcallis@google.com>
Reviewed-by: Seewai Fu <seewaifu@google.com>
diff --git a/api/controller/test.py b/api/controller/test.py
index 9856dee..0c75707 100644
--- a/api/controller/test.py
+++ b/api/controller/test.py
@@ -200,7 +200,7 @@
   allowed_chars = set(string.ascii_letters+string.digits+'-_.')
   invalid_chars = set(tag) - allowed_chars
   if invalid_chars:
-    return f'saw one or more invalid characters: {"".join(invalid_chars)}'
+    return f'saw one or more invalid characters: [{"".join(invalid_chars)}]'
 
   # Finally, max tag length is 128 characters
   if len(tag) > 128:
@@ -220,12 +220,12 @@
   allowed_chars = set(string.ascii_lowercase+string.digits+'-.')
   invalid_chars = set(key) - allowed_chars
   if invalid_chars:
-    return f'saw one or more invalid characters: {"".join(invalid_chars)}'
+    return f'saw one or more invalid characters: [{"".join(invalid_chars)}]'
 
   # Repeated . and - aren't allowed
   for char in '.-':
-    if char*2 in key:
-      return f'"{char}" can\'t be repeated in label key'
+    if char * 2 in key:
+      return f"'{char}' can\'t be repeated in label key" % char
 
 
 @faux.success(_BuildTestServiceContainersResponse)