Fix black Win32 smoke broken by my v5.21.1-11-g4077a6b

The problem with this was that I didn't test this on a system where $Q
was false. On those systems we not only warn about "Invalid conversion"
but also about the missing sprintf argument, as intended.

The test broke because the local __WARN__ handler would clobber the
"conversion" warning with the "missing" warning, the fix is easy, just
accumulate the warnings with ".=".

While poking at this I discovered a bug that's been here ever since the
test was added back in v5.10.0-1461-g53f65a9. If we emitted a warning on
systems where $Q was true we'd pass the test, this is because the empty
regex will match warnings, the test actually meant to use is() not
like(). It's also a bug that t/test.pl accepts non-regexes as the second
argument, Test::More doesn't:

    $ perl -MTest::More -wle 'like "", ""'
    not ok 1
    #   Failed test at -e line 1.
    #     '' doesn't look much like a regex to me.
    # Tests were run but no plan was declared and done_testing() was not seen.

This change might cause some additional black smoke because we'll now be
detecting warnings that we previously ignored, but I don't think this'll
fail on systems where $Q is true.

1 files changed, 11 insertions, 3 deletions

diff --git a/t/op/sprintf2.t b/t/op/sprintf2.t
index 5fd3cd7e95..4116237b60 100644
--- a/t/op/sprintf2.t
+++ b/t/op/sprintf2.t
@@ -297,9 +297,17 @@ my @tests = (
 for my $t (@tests) {
   my($fmt, $nums) = @$t;
   for my $num (@$nums) {
-    my $w; local $SIG{__WARN__} = sub { $w = shift };
-    is(sprintf($fmt, $num), $Q ? $num : $fmt, "quad: $fmt -> $num");
-    like($w, $Q ? '' : qr/Invalid conversion in sprintf: "$fmt"/, "warning: $fmt");
+    my $w = '';
+    local $SIG{__WARN__} = sub { $w .= shift };
+    my $sprintf_got = sprintf($fmt, $num);
+    if ($Q) {
+      is($sprintf_got, $num, "quad: $fmt -> $num");
+      is($w, '', "no warnings for: $fmt -> $num");
+    } else {
+      is($sprintf_got, $fmt, "quad unsupported: $fmt -> $fmt");
+      like($w, qr/Invalid conversion in sprintf: "$fmt"/, "got warning about invalid conversion from fmt : $fmt");
+      like($w, qr/Missing argument in sprintf/, "got warning about missing argument in sprintf from fmt : $fmt");
+    }
   }
 }