patch 9.2.0438: tests: test_plugin_termdebug is flaky

patch 9.2.0438: tests: test_plugin_termdebug is flaky

Problem:  Test_termdebug_tbreak(), Test_termdebug_basic(), and
          Test_termdebug_toggle_break() use synchronous assert_equal()
          to check breakpoint signs immediately after sending commands
          to gdb.  On slow CI (ASAN, ARM64, macOS) gdb may not have
          processed the response yet, causing the sign to be missing.
Solution: Wrap the three assertions in WaitForAssert() to poll until
          the signs are placed, matching the pattern already used by
          the other assertions in the same tests (Jesse Rosenstock).

closes: #20133

Co-authored-by: Gemini
Signed-off-by: Jesse Rosenstock <jmr@google.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>