[Polly][Test] Fix race condition while printing dot files.

[Polly][Test] Fix race condition while printing dot files.

The tests dot-scops.ll and dot-scops-npm.ll both wrote to the same file
scops.func.dot. If they are executed in parallel they will race for
the file. Fix by renaming func to func_npm in dot-scops-npm.ll so this
test writes dot scops.func_npm.dot.

Long-term, we will probably pass a file name (prefix) to the
printer pass such that we can use the guaranteed-unique LIT %t
placeholder in tests.