testsuite: ensure FLUX_HWLOC_XMLFILE works

Problem: No tests ensure that FLUX_HWLOC_XMLFILE works to cause the
resource module to use an XML file instead of topology load when
doing dynamic discovery.

Add a test to t0001-basic.t that ensures an instance started by
flux-start sees fake resources in an XML file when FLUX_HWLOC_XMLFILE
is set.

t/t0001-basic.t

@@ -368,6 +368,13 @@ test_expect_success 'flux-start dies gracefully when run from removed dir' '
 	 test_must_fail flux start /bin/true )
 '
 
+test_expect_success 'FLUX_HWLOC_XMLFILE works' '
+	lstopo --of xml -i "numa:2 core:3 pu:1" >test.xml &&
+	FLUX_HWLOC_XMLFILE=test.xml flux start -s 2 flux resource info \
+		>rinfo.out &&
+	test_debug "cat rinfo.out" &&
+	grep "12 Cores" rinfo.out
+'
 
 # too slow under ASAN
 test_expect_success NO_ASAN 'test_under_flux works' '