[MERGE #285] perftest.pl: fix a hard-coded path separator
[MERGE #285] perftest.pl: fix a hard-coded path separator

Merge pull request #285 from jianchun:perftest
"$dir\\$testcasename.js" does not work cross-platform. Replaced with
File::Spec->catfile call.

Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore