selftests/damon: Add executable permission to test scripts

When running the test for the damon subsystem, there are a lot of
warnings because test scripts do not have executable permission,
for example:

Warning: file debugfs_attrs.sh is not executable
Warning: file debugfs_schemes.sh is not executable
Warning: file debugfs_target_ids.sh is not executable
...

This patch adds executable permission to test scripts to eliminate
these warnings.
Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>