[flang][driver] Copy input files into a temp dir when testing

The following frontend driver invocation will generate 2 output files
in the same directory as the input files:
```
flang-new -fc1 input-1.f input-2.f
```
This is the desired behaviour. However, when testing we need to make
sure that we don't pollute the source directory. To this end, copy test
input files into a temporary directory.

Differential Revision: https://reviews.llvm.org/D94243