send-email tests: support GIT_TEST_PERL_FATAL_WARNINGS=true

Add support for the "GIT_TEST_PERL_FATAL_WARNINGS=true" test mode to
"send-email". This was added to e.g. git-svn in 5338ed2 (perl:
check for perl warnings while running tests, 2020-10-21), but not
"send-email". Let's rectify that.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

git-send-email.perl

@@ -18,7 +18,7 @@
 
 use 5.008;
 use strict;
-use warnings;
+use warnings $ENV{GIT_PERL_FATAL_WARNINGS} ? qw(FATAL all) : ();
 use POSIX qw/strftime/;
 use Term::ReadLine;
 use Getopt::Long;