Add tests for reportchars=a

Ref: #5066

Author: blueyed

testing/test_terminal.py

@@ -830,14 +830,20 @@ class Option(object):
     config.option.reportchars = "sfxw"
     assert getreportopt(config) == "sfx"
 
-    config.option.reportchars = "sfx"
+    # Now with --disable-warnings.
     config.option.disable_warnings = False
+    config.option.reportchars = "a"
+    assert getreportopt(config) == "sxXwEf"  # NOTE: "w" included!
+
+    config.option.reportchars = "sfx"
     assert getreportopt(config) == "sfxw"
 
     config.option.reportchars = "sfxw"
-    config.option.disable_warnings = False
     assert getreportopt(config) == "sfxw"
 
+    config.option.reportchars = "a"
+    assert getreportopt(config) == "sxXwEf"  # NOTE: "w" included!
+
     config.option.reportchars = "A"
     assert getreportopt(config) == "sxXwEfpP"