Add unit tests for self restart

[lchico]

Related issue
#438

Description

This PR introduces tests for the restart handler.

Tests

cd build/agent/restart_handler/tests/ && ctest -VV
UpdateCTestConfiguration  from :/root/wazuh-agent/build/agent/restart_handler/tests/DartConfiguration.tcl
UpdateCTestConfiguration  from :/root/wazuh-agent/build/agent/restart_handler/tests/DartConfiguration.tcl
Test project /root/wazuh-agent/build/agent/restart_handler/tests
Constructing a list of tests
Done constructing a list of tests
Updating test list for fixtures
Added 0 tests to meet fixture requirements
Checking test dependency graph...
Checking test dependency graph end
test 1
    Start 1: RestartHandler

1: Test command: /root/wazuh-agent/build/agent/restart_handler/tests/restart_handler_test
1: Working Directory: /root/wazuh-agent/build/agent/restart_handler/tests
1: Test timeout computed to be: 10000000
1: [==========] Running 5 tests from 3 test suites.
1: [----------] Global test environment set-up.
1: [----------] 2 tests from TestUsingSystemctl
1: [ RUN      ] TestUsingSystemctl.ReturnsTrueWhenSystemctlIsAvailable
1: [       OK ] TestUsingSystemctl.ReturnsTrueWhenSystemctlIsAvailable (1 ms)
1: [ RUN      ] TestUsingSystemctl.ReturnsFalseWhenSystemctlIsNotAvailable
1: [       OK ] TestUsingSystemctl.ReturnsFalseWhenSystemctlIsNotAvailable (0 ms)
1: [----------] 2 tests from TestUsingSystemctl (1 ms total)
1: 
1: [----------] 2 tests from StopAgentTest
1: [ RUN      ] StopAgentTest.GratefullyStop
1: [       OK ] StopAgentTest.GratefullyStop (2000 ms)
1: [ RUN      ] StopAgentTest.ForceStop
1: [       OK ] StopAgentTest.ForceStop (5000 ms)
1: [----------] 2 tests from StopAgentTest (7001 ms total)
1: 
1: [----------] 1 test from RestartWithForkTest
1: [ RUN      ] RestartWithForkTest.ShouldRestartUsingFork
1: [       OK ] RestartWithForkTest.ShouldRestartUsingFork (0 ms)
1: [----------] 1 test from RestartWithForkTest (0 ms total)
1: 
1: [----------] Global test environment tear-down
1: [==========] 5 tests from 3 test suites ran. (7003 ms total)
1: [  PASSED  ] 5 tests.
1/1 Test #1: RestartHandler ...................   Passed   11.01 sec

100% tests passed, 0 tests failed out of 1

Total Test time (real) =  11.01 sec

[MarcelKemp]

LGTM.