[5.1] lit.cfg: Respect the DYLD_LIBRARY_PATH restrictions when not using %target-run
Some test cases use something like
```
config.target_run_simple_swift = (
'%%empty-directory(%%t) && '
'%s %s %%s -o %%t/a.out -module-name main && '
'%s %%t/a.out &&'
'%s %%t/a.out’
```
That does not respect the %target-run DYLD_LIBRARY_PATH for choosing the just built libraries and/or OS libraries.
This PR respects said library path in such situations
rdar://problem/49835064