testsuite: avoid artificial corefile in test

Problem: In t3306-system-routercrash.t SIGSEGV is intentionally sent
to a broker to test a broker crash, but this could leave a mysterious
corefile in the test directory if the core file ulimit and system
configuration allow it.

Call `ulimit -c 0` before test_under_flux() to avoid the core file.

t/t3306-system-routercrash.t

@@ -15,6 +15,11 @@ This test verifies that the invariant holds when the router crashes.
 
 export TEST_UNDER_FLUX_TOPO=kary:1
 
+# A test below intentionally sends SIGSEGV to a broker process --
+# avoid an unnecessary corefile by ensuring the corefile limit is 0
+# before calling test_under_flux():
+ulimit -c 0
+
 test_under_flux 3 system
 
 startctl="flux python ${SHARNESS_TEST_SRCDIR}/scripts/startctl.py"