Copy input files to test directory.

With r196184, llvm-cov creates a new file right next to the input file.
However, the Inputs-directory can't simply be assumed to be writable
under all build systems.

Also, this prevents a new source file from showing up in the source tree
if the test aborts before the call to "rm".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196228 91177308-0d34-0410-b5e6-96231b3b80d8