test/py: improve regular expression for ut subtest symbol matcher

test/py: improve regular expression for ut subtest symbol matcher

Improve the regular expression that matches unittest symbols in
u-boot.sym.

Currently we do not enforce no prefix in symbol string, but with the
soon to come change in linker lists declaring lists and entries with the
__ADDRESSABLE macro (because of LTO), the symbol file will contain for
every symbol of the form
  _u_boot_list_2_ut_X_2_Y
also symbol
  __UNIQUE_ID___addressable__u_boot_list_2_ut_X_2_YN,
(where N at the end is some number).

In order to avoid matching these additional symbols, ensure that the
character before "_u_boot_list_2_ut" is not a symbol name character.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>