selftests: user_events: create test-specific Kconfig fragments

Create the config file in user_events directory of testcase which
need more kernel configuration than the default defconfig. User
could use these configs with merge_config.sh script:

The Kconfig CONFIG_USER_EVENTS=y is needed for the test to read
data from the following files,
- "/sys/kernel/tracing/user_events_data"
- "/sys/kernel/tracing/user_events_status"
- "/sys/kernel/tracing/events/user_events/*"

Enable config for specific testcase:
(export ARCH=xxx #for cross compiling)
./scripts/kconfig/merge_config.sh .config \
tools/testing/selftests/user_events/config

Enable configs for all testcases:
(export ARCH=xxx #for cross compiling)
./scripts/kconfig/merge_config.sh .config \
tools/testing/selftests/*/config

Cc: Beau Belgrave <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: [e-mail]
Signed-off-by: Naresh Kamboju <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/user_events/config

···
+CONFIG_USER_EVENTS=y