[ruby/strscan] Omit tests for `#scan_byte` and `#peek_byte` on

TruffleRuby temporary
(https://github.com/ruby/strscan/pull/91)

The methods were added in #89 but they aren't implemented in TruffleRuby
yet. So let's omit them for now to have CI green.

https://github.com/ruby/strscan/commit/844d963b56

test/strscan/test_stringscanner.rb

@@ -9,6 +9,7 @@ require 'test/unit'
 
 module StringScannerTests
   def test_peek_byte
+    omit("not implemented on TruffleRuby") if RUBY_ENGINE == "truffleruby"
     s = create_string_scanner('ab')
     assert_equal 97, s.peek_byte
     assert_equal 97, s.scan_byte
@@ -19,6 +20,7 @@ module StringScannerTests
   end
 
   def test_scan_byte
+    omit("not implemented on TruffleRuby") if RUBY_ENGINE == "truffleruby"
     s = create_string_scanner('ab')
     assert_equal 97, s.scan_byte
     assert_equal 98, s.scan_byte