testsuite: reduce parallelism in t4000-issues-test-driver.t

Problem: The issues tests in t4000-issues-test-driver.t are run as
jobs in a test instance of size=2, but the instance inherits the
total number of cores from the system, which means up to 2*ncores
issues tests could be run at the same time. On some systems, this
causes failures in the tests due to overloading the system.

Switch to the "job" personality of test_under_flux, which will
configure the test instance with only 2 fake cores per rank, keeping
the parallelism of the tests to 4, which is probably a safe number.

Fixes #6856

Author: grondo

t/t4000-issues-test-driver.t

@@ -10,8 +10,13 @@ if test_have_prereq ASAN; then
     test_done
 fi
 
+# Note: use test_under_flux "job" personality so that only 2 cores per fake
+# node are configured in the test instance. This ensures a maximum of 4
+# issues test scripts (invoked as jobs below) run simultaneously, instead
+# of 1 per real core, which could cause test failures on overloaded systems.
+#
 SIZE=2
-test_under_flux ${SIZE}
+test_under_flux ${SIZE} job
 echo "# $0: flux session size will be ${SIZE}"
 
 if test -z "$T4000_ISSUES_GLOB"; then