t/porting/header_parser.t: un-TODO warnings test

The absorbtion feature may not be implemented yet, but the current code
doesn't throw any warnings either, so don't mark the "no warnings" test
as TODO. This makes test runs a bit less noisy:

    Test Summary Report
    -------------------
    porting/header_parser.t    (Wstat: 0 Tests: 37 Failed: 0)
      TODO passed:   31

Author: mauke

t/porting/header_parser.t

@@ -981,22 +981,24 @@ EOF_BLURB
 {
     my @warn;
     local $SIG{__WARN__}= sub { push @warn, $_[0]; warn $_[0] };
-    local $::TODO;
-    $::TODO= "Absorbtion not implemented yet";
-    # currently we don't handle absorbtion: (A && (A || B || C ...)) == A
-    my $hp= HeaderParser->new(debug=>0,add_commented_expr_after=>0);
-    $hp->parse_text(<<~'EOF');
-        #if defined(X) && (defined(X) || defined(Y))
-        #define HAS_X
-        #endif
-        EOF
-    my $grouped= $hp->group_content();
-    my $as_text= $hp->lines_as_str($grouped);
-    is($as_text,<<~'EOF',my $tname= "simplification by absorbtion"); # or show_text($as_text);
-        #if defined(X)
-        # define HAS_X
-        #endif /* defined(X) */
-        EOF
+    my $tname = "simplification by absorbtion";
+    {
+        local $::TODO = "Absorbtion not implemented yet";
+        # currently we don't handle absorbtion: (A && (A || B || C ...)) == A
+        my $hp= HeaderParser->new(debug=>0,add_commented_expr_after=>0);
+        $hp->parse_text(<<~'EOF');
+            #if defined(X) && (defined(X) || defined(Y))
+            #define HAS_X
+            #endif
+            EOF
+        my $grouped= $hp->group_content();
+        my $as_text= $hp->lines_as_str($grouped);
+        is($as_text,<<~'EOF', $tname); # or show_text($as_text);
+            #if defined(X)
+            # define HAS_X
+            #endif /* defined(X) */
+            EOF
+    }
     is(join("\n",@warn),"", "No warnings generated from $tname");
 }
 {