Check that tests use strict and warnings

t/repo.pl wasn't using strict/warnings, causing me to miss some issues
with cleanup.

xt/strict.t

@@ -0,0 +1,28 @@
+#!/usr/bin/perl
+
+# Check that test files and test libs are using string and warnings.
+
+use strict;
+use warnings FATAL => 'all';
+
+use Test::More;
+
+sub is_strict_and_warn {
+    my $filename = shift;
+    my ($is_strict, $is_warn);
+    open my $fh, '<', $filename or die "check $filename: $!";
+    for (<$fh>) {
+        m/^use strict;/ and $is_strict = 1;
+        m/^use warnings/ and $is_warn = 1;
+        if ($is_strict && $is_warn) {
+            return 1;
+        }
+    }
+    return 0;
+}
+
+my @filenames = glob('t/*.t t/*.pl xt/*.t xt/*.pl');
+plan tests => scalar(@filenames);
+for my $fn (@filenames) {
+    ok(is_strict_and_warn($fn), "$fn is using strict and warnings");
+}