testsuite: Pipe stdin directly to process

Previously the driver would read the stdin content from the source file
and then write it to the subprocess' stdin. We now simply open the stdin
file and provide that handle to the subprocess as its stdin

Test Plan: Validate

Reviewers: austin

Subscribers: rwbarton, thomie, goldfire

Differential Revision: https://phabricator.haskell.org/D3735