t2802-uri-cmd.t: add test for PID resolver on broker with no child

Problem: The URI resolver tests do not exercise an error condition
where the PID target is a flux-broker with no children. This let a
bug creep through undetected.

Add a test to ensure that the PID URI resolver on a flux-broker
process with no children raises an error such as:

 flux-uri: ERROR: PID 28628 is a flux-broker and no child found

Author: grondo

t/t2802-uri-cmd.t

@@ -4,7 +4,7 @@ test_description='Test flux uri command'
 
 . $(dirname $0)/sharness.sh
 
-test_under_flux 1
+test_under_flux 2
 
 testssh="${SHARNESS_TEST_SRCDIR}/scripts/tssh"
 
@@ -51,6 +51,11 @@ test_expect_success 'flux-uri pid resolver works on flux-broker with fallback' '
 	  test "$(flux uri pid:$(flux getattr broker.pid))" =  "$FLUX_URI"
 	)
 '
+test_expect_success 'flux-uri pid resolver fails if broker has no child' '
+	test_must_fail flux uri pid:$(flux exec -r 1 flux getattr broker.pid) \
+		>broker-no-child.out 2>&1 &&
+	grep "is a flux-broker and no child found" broker-no-child.out
+'
 test_expect_success 'flux-uri pid resolver fails for nonexistent pid' '
 	test_expect_code 1 flux uri pid:123456
 '