tp: relax strict upper bound check in TestBackoffBeforeDeadline

[Aditya-9-6]

Problem

In tp/client_test.go, TestBackoffBeforeDeadline/backoff_sleeps_past_deadline creates a context with a 10-second timeout:

ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()

bo := boBeforeDeadline(ctx, time.Hour)
assert.True(t, bo > 8*time.Second, "got %s", bo)
assert.True(t, bo < 9*time.Second, "got %s", bo)

boBeforeDeadline computes:

remaining := time.Until(deadline) // ~10s
slack := remaining / 2
if slack > pollDeadlineSlack { // pollDeadlineSlack = 1s
    slack = pollDeadlineSlack
}
latest := remaining - slack // 10s - 1s = 9s

When the test executes rapidly and negligible time has elapsed between context creation and time.Until(deadline), remaining is 10s and latest evaluates to exactly 9s.
Because the assertion checks strict inequality (bo < 9*time.Second), it intermittently fails with:

client_test.go:42: got 9s

Solution

Change the upper bound assertion to assert.True(t, bo <= 9*time.Second, "got %s", bo). This permits the boundary value of 10s - 1s = 9s when time.Until(deadline) has not yet ticked below 10s, while preserving the test invariant that bo is clamped to roughly remaining - pollDeadlineSlack.