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 #50

tests/fsplit-p.tst

@@ -10,18 +10,15 @@ __IN__
 ]
 __OUT__
 
-(
-export IFS=X
-
 test_o 'default IFS (overriding environment variable)'
+IFS=X $TESTEE <<\__INNER__
 printf "[%s]\n" "$IFS"
+__INNER__
 __IN__
 [ 	
 ]
 __OUT__
 
-)
-
 test_oE -e 0 'field splitting applies to results of expansions'
 IFS=' 0' a='1 2'
 bracket     -${a}- -$(echo '3 4')- -`echo '5 6'`- -$((708))-