roachtest: run fewer TPHC imports, increase timeout

The TPCH import roachtest has been timing out for quite a while. Running it by hand, it appears it
_is_ making progress when it times out, which suggests it has become slower since the timeouts were
picked, which seems to be supported by comments claiming it used to run in 3h with 8 nodes. Exactly
why it is slower and how to fix it is something that will require further study, but in the meantime
its constant failures aren't useful and contribute to overall test flakiness. Until we have a chance
to dig into this further, this removes the 4 and 32 node configurations and gives the remaining 8-node
test 50% longer to complete.

Release note: none.

pkg/cmd/roachtest/import.go

@@ -111,9 +111,15 @@ func registerImportTPCH(r *testRegistry) {
 		nodes   int
 		timeout time.Duration
 	}{
-		{4, 6 * time.Hour}, // typically 4-5h
-		{8, 4 * time.Hour}, // typically 3h
-		{32, 3 * time.Hour},
+		// TODO(dt): this test seems to have become slower as of 19.2. It previously
+		// had 4, 8 and 32 node configurations with comments claiming they ran in in
+		// 4-5h for 4 node and 3h for 8 node. As of 19.2, it seems to be timing out
+		// -- potentially because 8 secondary indexes is worst-case for direct
+		// ingestion and seems to cause a lot of compaction, but further profiling
+		// is required to confirm this. Until then, the 4 and 32 node configurations
+		// are removed (4 is too slow and 32 is pretty expensive) while 8-node is
+		// given a 50% longer timeout (which running by hand suggests should be OK).
+		{8, 6 * time.Hour},
 	} {
 		item := item
 		r.Add(testSpec{