testsuite: obey FLUX_TESTS_LOGFILE in t2315-resource-system.t

Problem: t2315-resource-system.t doesn't generate a logfile when
FLUX_TESTS_LOGFILE is set because it doesn't use test_under_flux()
and doesn't otherwise specifically check for this variable.

Check for FLUX_TESTS_LOGFILE and add the appropriate --logfile
option to sharness when it is set.

Author: grondo

t/t2315-resource-system.t

@@ -2,6 +2,8 @@
 
 test_description='Test resource module with system instance config'
 
+# Append --logfile option if FLUX_TESTS_LOGFILE is set in environment:
+test -n "$FLUX_TESTS_LOGFILE" && set -- "$@" --logfile
 . `dirname $0`/sharness.sh
 
 if which hwloc-bind > /dev/null; then