Make test target name unique.

[gberardi-pillar]

everest-core cannot build with tests (BUILD_TESTING=ON) due to submodules having generic names that collide when CMake runs.

This change makes the test target for libevse-security unique by naming it after the repo, a pattern that can be followed for all repos.

NOTE: This change does not enable automated builds and tests, as there are other PRs to handle that work.
I will also point out that when I run the tests manually, 17 out of 26 tests fail.

ALSO NOTE: I changed the flag from BUILD_TESTING_EVSE_SECURITY to BUILD_TESTING, since that variable gets used by CTest and is needed to allow the make test target to work.

I also used the Modern CMake pattern of enabling tests only if the project is the main project (not a child of a parent project) or if it is explicitly specified with cmake -Deverest-evse_security_BUILD_TESTING=ON to allow flexibility.

[gberardi-pillar]

@shankari Here's a minor change to review.

[gberardi-pillar]

@valentin-dimov Since you have contributed the most to this repo, would you be able to look at this PR?

[Dominik-K]

Let's discuss in general at

• Test-target names unique and don't build tests of dependencies (by default) EVerest#129

[shankari]

I don't understand the reason for the comment
#28 (comment)
but I have asked that as a question in
EVerest/EVerest#129 (comment)

I don't see any other questions here, and it would be good to make more progress on cleaning up the tests.

I approve assuming that you will address the comment assuming that it is warranted by the outcome of the larger discussion before merge.