selftest: hid: add the missing tests directory

Commit 160c826b4dd0 ("selftest: hid: add missing run-hid-tools-tests.sh")
has added the run-hid-tools-tests.sh script for it to be installed, but
I forgot to add the tests directory together.

If running the test case without the tests directory, will results in
the following error message:

make -C tools/testing/selftests/ TARGETS=hid install \
INSTALL_PATH=$KSFT_INSTALL_PATH
cd $KSFT_INSTALL_PATH
./run_kselftest.sh -t hid:hid-core.sh

/usr/lib/python3.11/site-packages/_pytest/config/__init__.py:331: PluggyTeardownRaisedWarning: A plugin raised an exception during an old-style hookwrapper teardown.
Plugin: helpconfig, Hook: pytest_cmdline_parse
UsageError: usage: __main__.py [options] [file_or_dir] [file_or_dir] [...]
__main__.py: error: unrecognized arguments: --udevd
inifile: None
rootdir: /root/linux/kselftest_install/hid

In fact, the run-hid-tools-tests.sh script uses the scripts in the tests
directory to run tests. The tests directory also needs to be added to be
installed.

Fixes: ffb85d5c9e80 ("selftests: hid: import hid-tools hid-core tests")
Cc: [e-mail]
Signed-off-by: Yun Lu <[e-mail]>
Acked-by: Benjamin Tissoires <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/hid/Makefile

···
 TEST_PROGS += hid-wacom.sh
 
 TEST_FILES := run-hid-tools-tests.sh
+TEST_FILES += tests
 
 CXX ?= $(CROSS_COMPILE)g++