[#3831] Added path validation, updated tests

[#3831] Added path validation, updated tests

/src/hooks/dhcp/forensic_log/libloadtests/load_unload_unittests.cc
    Updated tests

/src/hooks/dhcp/forensic_log/rotating_file.cc
    RotatingFile::apply{) - fetch default from singleton

/src/hooks/dhcp/forensic_log/tests/legal_log_mgr_unittests.cc
    TEST_F(LegalLogMgrTest, pathValidation)
    TEST_F(LegalLogMgrTest, pathEnvVarOverride) - new tests

/src/lib/dhcpsrv/legal_log_mgr.*
    LegalLogMgr::parseFile() - validate path

Updated the ARM.