GEDCOM-import tests explicitly set the locale to en_US.UTF8 because they
assume a US date and time format. That locale is not guaranteed to be
present and checking that in ebuild is a hassle, therefore use C.UTF8
instead - it's similar enough.

--- a/data/tests/imp_sample.gramps
+++ b/data/tests/imp_sample.gramps
@@ -1706,7 +1706,7 @@
       </style>
     </note>
     <note handle="_000000f9000000f9" change="1591544255" id="N0036" type="General">
-      <text>Objects referenced by this note were missing in a file imported on 12/25/1999 12:00:00 AM.</text>
+      <text>Objects referenced by this note were missing in a file imported on 12/25/99 00:00:00.</text>
     </note>
   </notes>
 </database>
--- a/gramps/plugins/test/imports_test.py
+++ b/gramps/plugins/test/imports_test.py
@@ -51,10 +51,7 @@
 # ------------------------------------------------------------------
 
 # These tests assume a US date and time format.
-try:
-    locale.setlocale(locale.LC_ALL, 'en_US.utf8')
-except locale.Error:     # seems to fail on Windows system for some reason
-    locale.setlocale(locale.LC_ALL, 'English_United States')
+locale.setlocale(locale.LC_ALL, 'C.utf8')
 
 
 def mock_time(*args):