Mark t/nqp/063-slurp.t as self-referential

Also, there is no need to accept two numbers as the number of chars
anymore. With 0412c22 the JVM backend learned to count a CRLF
as one char as well. So even if the test file would be written with
CRLF as line endings, nqp::chars should give back the same number
as for LF as line endings.

This should fix #512.

t/nqp/063-slurp.t

@@ -1,6 +1,8 @@
+# This test file is self-referential and shouldn't be changed.
+# Please put other tests for slurp in another file.
 plan(1);
 
 my $content := slurp("t/nqp/063-slurp.t");
-ok(nqp::chars($content) == 150 || nqp::chars($content) == 156, "File slurped");
+ok(nqp::chars($content) == 234, "File slurped");
 
 # vim: ft=perl6