testsuite: update usage of flux-ping --pad option

Problem: Tests in t0007-ping.t use the full number of bytes with the
pad option, but this unnecessary and verbose.

Update the uses of the --pad option to include a K or M suffix on
the argument for readability.

Author: grondo

t/t0007-ping.t

@@ -17,23 +17,23 @@ test_expect_success 'ping: 10K 1K byte echo requests' '
 '
 
 test_expect_success 'ping: 1K 10K byte echo requests' '
-	run_timeout 15 flux ping --pad 10240 --count 1024 --interval 0 0
+	run_timeout 15 flux ping --pad 10K --count 1024 --interval 0 0
 '
 
 test_expect_success 'ping: 100 100K byte echo requests' '
-	run_timeout 15 flux ping --pad 102400 --count 100 --interval 0 0
+	run_timeout 15 flux ping --pad 100K --count 100 --interval 0 0
 '
 
 test_expect_success 'ping: 10 1M byte echo requests' '
-	run_timeout 15 flux ping --pad 1048576 --count 10 --interval 0 0
+	run_timeout 15 flux ping --pad 1M --count 10 --interval 0 0
 '
 
 test_expect_success 'ping: 10 1M byte echo requests (batched)' '
-	run_timeout 15 flux ping --pad 1048576 --count 10 --batch --interval 0 0
+	run_timeout 15 flux ping --pad 1M --count 10 --batch --interval 0 0
 '
 
 test_expect_success 'ping: 1K 10K byte echo requests (batched)' '
-	run_timeout 20 flux ping --pad 10240 --count 1024 --batch --interval 0 0
+	run_timeout 20 flux ping --pad 10K --count 1024 --batch --interval 0 0
 '
 
 test_expect_success 'ping --rank 1 works' '