RFR [6943190] TEST_BUG: java/lang/Runtime/exec/ExecWithInput.java hardcodes path to cat

[Ivan Gerasimov]

Thank you Alan!

> This looks okay although I would prefer for a number of these tests to 
> fail if the command is not found (otherwise it is will just hide issues).

Now I've got two suggestions that somehow contradict each other.
Martin suggested to check for particular OS only as a last resort, but 
without knowing that we run under Unix, we cannot treat a command 
absence as an error.

I think we can assume that none of the tests which need UnixCommands are 
for Windows, so I added explicit checking for that.
Having made sure the OS is a Unix (i.e. not Windows), the absence of a 
required command now causes an exception to be thrown.

Would you please take a look at the updated webrev?

http://cr.openjdk.java.net/~igerasim/6943190/6/webrev/

Sincerely yours,
Ivan