t/test-emails: invoke $MULTIMAIL with fixed $USER

Since the output of $MULTIMAIL depends on $USER, set it to a temporary
constant value before invoking the command.  This will make it easier to
compare expected output with actual output.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>

t/test-emails

@@ -15,7 +15,7 @@ test-email() {
     REFNAME="$1"
     OLDREV="$2"
     NEWREV="$3"
-    echo "$OLDREV" "$NEWREV" "$REFNAME" | "$MULTIMAIL"
+    echo "$OLDREV" "$NEWREV" "$REFNAME" | USER=vanille "$MULTIMAIL"
 }
 
 test-create() {