tests: support setting/using blank content env variables

- test450: remove --config from the keywords
- test2080: change return code
- test428: add --config as a keyword
- test428: disable on Windows due to CI problems

tests/data/test428

@@ -3,6 +3,7 @@
 <keywords>
 HTTP
 variables
+--config
 </keywords>
 </info>
 
@@ -28,6 +29,14 @@ Funny-head: yesyes
 #
 # Client-side
 <client>
+
+# For unknown reasons, a number of CI jobs on Appveyor keep returning NULL to
+# getenv() for the blank environment variable which makes the test fail.
+# Unfortunately, this makes me disable the test completely on Windows.
+
+<features>
+!win32
+</features>
 <server>
 http
 </server>

tests/data/test450

@@ -2,7 +2,6 @@
 <info>
 <keywords>
 HTTP
---config
 variables
 </keywords>
 </info>