Out of the tree build fails several tests executing "codon_test"

When building this project out of tree (for example in /tmp) and then executing `codon_test` several tests fail due to somehow assuming the source tree is at `../` :
```
mkdir /tmp/codon-build
cd /tmp/codon-build
cmake -S $codon_source_path \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_DIR=$(llvm-config --cmakedir) \
-DCMAKE_C_COMPILER=clang \
-DCMAKE_CXX_COMPILER=clang++
...
cmake --build . --config Release
...
./codon_test
...
446 FAILED TESTS
```
But if moving `codon_test` and it's dependencies under a folder inside the source tree then the tests pass.