t/tr.t: Add test case

While debugging, I noticed that some code in doop.c:S_trans_simple() was
not getting called by anything in our test suite; this new test does so.

t/op/tr.t

@@ -14,7 +14,7 @@ BEGIN {
 use utf8;
 require Config;
 
-plan tests => 317;
+plan tests => 318;
 
 # Test this first before we extend the stack with other operations.
 # This caused an asan failure due to a bad write past the end of the stack.
@@ -1211,4 +1211,14 @@ for ("", nullrocow) {
     is $c, 1, "Count for the above test";
 }
 
+{   # As of August 2024, the code to handle this situation is not otherwise
+    # exercised by the test suite
+
+    my $A = "A";
+    utf8::upgrade($A);
+
+    $A =~ tr/A/\xB6/;   # B6 works in both ASCII and EBCDIC
+    is($A, "\xB6", "UTF-8 invariant in a UTF-8 string to UTF-8 variant");
+}
+
 1;