t: cover large parallel fsck

Problem: flux-fsck now checks valref blobrefs in parallel but there
are no tests to stress this.

Add a large 2000 element valref blobref array into the tests in t2816-fsck-cmd.t.

Author: chu11

t/t2816-fsck-cmd.t

@@ -50,6 +50,28 @@ test_expect_success 'flux-fsck verbose works' '
 	grep "dir\.b" verbose.out &&
 	grep "alink" verbose.out
 '
+# Cover value with a very large number of appends
+# N.B. from 1000 to 3000 instead of 0 to 2000, easier to debug errors
+# using fold(1) (i.e. all numbers same width)
+test_expect_success LONGTEST 'load kvs and create some kvs content' '
+       flux module load kvs &&
+       for i in `seq 1000 3000`; do
+	   flux kvs put --append bigval=${i}
+       done &&
+       flux kvs get bigval > bigval.exp
+'
+test_expect_success LONGTEST 'call sync to ensure we have checkpointed' '
+	flux kvs sync
+'
+test_expect_success LONGTEST 'unload kvs' '
+       flux module remove kvs
+'
+test_expect_success LONGTEST 'flux-fsck works' '
+	flux fsck --verbose 2> bigval.out &&
+	grep "Checking integrity" bigval.out &&
+	grep "bigval" bigval.out &&
+	grep "Total errors: 0" bigval.out
+'
 test_expect_success 'load kvs' '
 	flux module load kvs
 '