client.Command and client.FileCommand improvements

[Jaymon]

path = testdata.create_file(contents=[
    "#!{}".format(testdata.get_interpreter()),
    "print('hello')
])

s = testdata.FileCommand(path)

Didn't work because FileCommand added the .py suffix, it would be nice to fix that by being able to pass in a value to the FileCommand.__init__() method.

So I switched it to:

s = testdata.Command(path)

But that failed because path didn't have the right permissions, so it might be worth having the files be default 777 or having an easy way to set these permissions, even if it was a create_script() method.

What finally worked was:

s = testdata.Command("{} {}".format(testdata.get_interpreter(), path))

Which doesn't seem ideal.