Mask object addresses in message assertions

[hsbt]

Eight tests in test/test_error_highlight.rb fail intermittently under a moving GC. The expected message heredoc evaluates the receiver's #inspect at the call site and bakes in the object address, but the actual message is only built later, when #message runs inside the block. A GC that relocates the receiver in that window makes the two addresses disagree. Seven of them keep a singleton on the receiver, so name_err_mesg_to_str takes the rb_any_to_s branch and prints an address regardless of the message format. test_local_variable_get reaches the same place through rb_inspect on a Binding.

I normalize both sides in assert_error_message through a new mask_addresses, which rewrites 0x\h+ to 0xXXXX. Doing it in the helper covers all eight sites and any future assertion of the same shape, so the individual #inspect calls stay as they are.

Forcing the relocation with GC.verify_compaction_references fails all eight before the change and none after. This is a forward-port of ruby/ruby#18731, and the file stays byte-identical to test/error_highlight/test_error_highlight.rb upstream.

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