[flang][driver] Copy test file into a temp dir when testing (nfc)

The following driver invocation will generate an output file
in the same directory as the input file:

```
flang-new -fc1 -test-io test-input.f90
```

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 the
input file into a temporary directory before testing.

This is similar to https://reviews.llvm.org/D94243.