wasm-reduce: don't require test file and working file

IIUC:

• -t provides the test file, which is what the predicate script is supposed to test
• -w provides the working file, which is where the final reduced result (and current-smallest during reduction) is placed

Users must provide both or wasm-reduce will not run.

I think there would be better user ergonomics if the predicate script was either

• passed the file to test as an argument, or
• the predicate script was always invoked with a temp dir as the current dir and that temp dir has the file to test inside it so that if the predicate script uses the test file via a relative path it always works (this is what creduce does)

Given one of those choices, then the user would only need to supply the input file and the predicate script. They would not also need to supply the test file and working file.

Additionally, both of those approaches would allow for multiple executions of the predicate script in parallel, opening up opportunities for speeding up the reduction process.

Start by reading the wasm-reduce command behavior around the -t and -w options, then compare the proposed predicate-script invocation approaches with the current workflow. Define the intended input and output arguments and verify that reduction can run with only the input file and predicate script, including parallel predicate executions.