btrace: add unsupported/untested messages when skipping tests

We may silently skip gdb.btrace tests if

  - the target does not support record-btrace
  - the target does not support TSX
  - the target does not support gdbserver
  - we fail to compile the test
  - we fail to run to main

Add unsupported/untested messages for each of those.

testsuite/
	* gdb.btrace/buffer-size.exp: Add unsupported/untested message if
	the test is skipped.
	* gdb.btrace/data.exp: Likewise.
	* gdb.btrace/delta.exp: Likewise.
	* gdb.btrace/dlopen.exp: Likewise.
	* gdb.btrace/enable-running.exp: Likewise.
	* gdb.btrace/enable.exp: Likewise.
	* gdb.btrace/exception.exp: Likewise.
	* gdb.btrace/function_call_history.exp: Likewise.
	* gdb.btrace/gcore.exp: Likewise.
	* gdb.btrace/instruction_history.exp: Likewise.
	* gdb.btrace/multi-thread-step.exp: Likewise.
	* gdb.btrace/nohist.exp: Likewise.
	* gdb.btrace/non-stop.exp: Likewise.
	* gdb.btrace/reconnect.exp: Likewise.
	* gdb.btrace/record_goto-step.exp: Likewise.
	* gdb.btrace/record_goto.exp: Likewise.
	* gdb.btrace/rn-dl-bind.exp: Likewise.
	* gdb.btrace/segv.exp: Likewise.
	* gdb.btrace/step.exp: Likewise.
	* gdb.btrace/stepi.exp: Likewise.
	* gdb.btrace/tailcall-only.exp: Likewise.
	* gdb.btrace/tailcall.exp: Likewise.
	* gdb.btrace/tsx.exp: Likewise.
	* gdb.btrace/unknown_functions.exp: Likewise.
	* gdb.btrace/vdso.exp: Likewise.