Test malformed utf8 filenames roundtripping properly

Fixes RT#127671. Please feel free to object these tests (using similar
concerns raised in the ticket). However, I still can't see how
something in a subdirectory of /tmp can create a problem for any
user. To me it sounds like nothing should ever touch or see these
files anyway.

S32-str/utf8-c8.t

@@ -5,7 +5,7 @@ use Test;
 # 8-bit octet stream given to us by OSes that don't promise anything about
 # the character encoding of filenames and so forth.
 
-plan 59;
+plan 63;
 
 {
     my $test-str;
@@ -177,6 +177,26 @@ is Buf.new(0xFE).decode('utf8-c8').chars, 1, 'Decoding Buf with just 0xFE works'
     $fh.close;
 }
 
+# RT#127671
+if $*DISTRO.is-win {
+    skip('Not clear if there is an alternative to this issue on Windows', 4);
+} else {
+    my $test-dir = $*TMPDIR ~ '/tmp.' ~ $*PID ~ '-' ~ time;
+    mkdir $test-dir;
+    # ↑ normal directory in TMPDIR to hide our scary stuff
+    my $file = ("$test-dir/".encode ~ Buf.new(0x06, 0xAB)).decode('utf8-c8');
+    # ↑ a file with a name that is somewhat weird
+    END { try unlink $file; try rmdir $test-dir }
+    spurt $file, 'hello'; # create the file
+    lives-ok {
+        my @files = $test-dir.IO.dir;
+        is @files.elems, 1, 'dir returns something when encountering malformed utf8 names';
+        my $roundtrip = @files[0].basename.encode('utf8-c8');
+        is-deeply $roundtrip.list, (0x06, 0xAB), 'utf8-c8 name roundtripped through dir';
+        is @files.IO.slurp, 'hello', 'can slurp a file with utf8-c8 name';
+    }, 'dir lives with malformed utf8 names';
+}
+
 # MoarVM #664
 # UTF8-C8 string changes under concatenation/substr
 {