Tests: Add require.Eventually and fix a few test flakes (#16217)

**What type of PR is this?**

Other

**What does this PR do? Why is it needed?**

This is a better way to wait for a test condition to hit, rather than
time.Sleep.

**Which issues(s) does this PR fix?**


**Other notes for review**

**Acknowledgements**

- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).

testing/require/requires.go

@@ -1,6 +1,8 @@
 package require
 
 import (
+	"time"
+
 	"github.com/OffchainLabs/prysm/v7/testing/assertions"
 	"github.com/sirupsen/logrus/hooks/test"
 )
@@ -87,3 +89,9 @@ func ErrorIs(tb assertions.AssertionTestingTB, err, target error, msg ...any) {
 func StringContains(tb assertions.AssertionTestingTB, expected, actual string, msg ...any) {
 	assertions.StringContains(tb.Fatalf, expected, actual, true, msg)
 }
+
+// Eventually asserts that given condition will be met within waitFor time,
+// periodically checking target function each tick.
+func Eventually(tb assertions.AssertionTestingTB, condition func() bool, waitFor, tick time.Duration, msg ...any) {
+	assertions.Eventually(tb.Fatalf, condition, waitFor, tick, msg...)
+}