[CTS] Fix UR_CONFORMANCE_ENABLE_MATCH_FILES=OFF

[frasercrmck]

When configured without match files, the CTS was trying to run a command and its arguments as a single command, as it was represented by a single string variable. It needs to be a single string in the default case - when building with UR_CONFORMANCE_ENABLE_MATCH_FILES=ON - as that command string is passed off to another command via a flag, so it must be seen by the shell as one item.

While definitely not nice, this is the simplest fix I could think of which preserves the behaviour of the (more important, tested) 'default' case.