Higher grace period for deadline propagation (#782)

This is to combat flaky tests. The existing threshold of 100ms is not
always sufficient. Here's a recent failure, where the actual delta was
135ms:
https://github.com/connectrpc/conformance/actions/runs/7832580880/job/21371604793?pr=781
So this increases the threshold to 250ms.

internal/app/connectconformance/results.go

@@ -36,7 +36,10 @@ import (
 	"google.golang.org/protobuf/types/known/anypb"
 )
 
-const timeoutCheckGracePeriodMillis = 100
+// CI environments have been observed to have 150ms of delay between the deadline
+// being set and it being serialized in a request header. This adds a little extra
+// headroom to avoid flaky tests.
+const timeoutCheckGracePeriodMillis = 250
 
 // testResults represents the results of running conformance tests. It accumulates
 // the state of passed and failed test cases and also reports failures to a given