testsuite: t0007-ping.t: do not assume short hostnames

Problem: A couple tests in `t0007-ping.t` assume short hostnames
by using $(hostname -s) as an expected value. This causes failures
when the domain is part of the hostname.

Change uses of $(hostname -s) to just $(hostname)

Fixes #4109

t/t0007-ping.t

@@ -138,10 +138,10 @@ test_expect_success 'ping help output works' '
 '
 
 test_expect_success 'ping works with hostname' '
-        flux ping --count=1 $(hostname -s)
+        flux ping --count=1 $(hostname)
 '
 test_expect_success 'ping works with hostname!service' '
-        flux ping --count=1 "$(hostname -s)!broker"
+        flux ping --count=1 "$(hostname)!broker"
 '
 test_expect_success 'ping fails with unknown hostname' '
         test_must_fail flux ping --count=1 "notmyhost!broker"