Move of_grad_cd.csv.ref from TestCases repo to TestCases/ folder

[martin-g]

Proposed Changes

Move radiation/p1adjoint/of_grad_cd.csv.ref from https://github.com/su2code/TestCases.git to TestCases/ folder

Discussion: https://su2devteam.slack.com/archives/C2A1JLGDR/p1654254077647149

Related Work

su2code/TestCases#102

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags, or simply --warnlevel=2 when using meson).
• My contribution is commented and consistent with SU2 style.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.