Don't export IFS outside testcase function

Don't export IFS outside testcase function

The run-test.sh script depends on field splitting to pass the correct
option to the diff utility in the testcase function. This means the
function should be called with the default IFS. This commit modifies the
test case that needs a non-default IFS to export it only inside the test
case script.

Fixes https://github.com/magicant/yash/issues/50