Tests: fix: list_triggers_cli: kallsyms contains prefixed symbols

Tests: fix: list_triggers_cli: kallsyms contains prefixed symbols

The test_list_triggers_cli test fails with:

  # Listing event-rule-matches kernel probe
  ./tests/regression/tools/trigger/test_list_triggers_cli: line 808: [[: 0xffffffffc1fca020
  ffffffffc1fca030: syntax error in expression (error token is "ffffffffc1fca030")
  ./tests/regression/tools/trigger/test_list_triggers_cli: line 813: 0xffffffffc1fca020
  ffffffffc1fca030 - 0xffffffffc1fca0c0
  ffffffffc1fca0b0 : syntax error in expression (error token is "ffffffffc1fca030 - 0xffffffffc1fca0c0
  ffffffffc1fca0b0 ")

This is due to the fact that /proc/kallsyms now contains prefixed
symbols (e.g. `__pfx_lttng_channel_enable`) which cause the test to fail
since the grep invocation doesn't expect more than one match.

Matching the begin/end of the "word" (or rather, function name) allows
us to check for exact matches only.

https://github.com/torvalds/linux/commit/9f2899fe36a623885d8576604cb582328ad32b3c

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ifc04757719c733caf55c8eea447e18a2246a9f49