dashboard: shard linux-amd64-longtest-race and windows-amd64-longtest-newcc builders

Fixes golang/go#56907.

Change-Id: Iede30e41588326d6922f95c7c2dc1e837d801cfc
Reviewed-on: https://go-review.googlesource.com/c/build/+/452857
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: cherrymui

dashboard/builders.go

@@ -1910,6 +1910,7 @@ func init() {
 		env: []string{
 			"GO_TEST_TIMEOUT_SCALE=5", // Inherited from the longtest builder.
 		},
+		numTryTestHelpers: 4, // Target time is < 15 min for go.dev/issue/56907.
 	})
 	addBuilder(BuildConfig{
 		Name:     "linux-386-longtest",
@@ -2314,7 +2315,8 @@ func init() {
 		env: []string{
 			"GO_TEST_TIMEOUT_SCALE=5", // give them lots of time
 		},
-		KnownIssues: []int{35006},
+		KnownIssues:       []int{35006},
+		numTryTestHelpers: 4, // Target time is < 15 min for go.dev/issue/42661.
 	})
 	addBuilder(BuildConfig{
 		Name:     "windows-amd64-race",