Fix flaky tests

[iRevive]

There are a few:

• SdkTracerSuite

Example

Job example: https:/iRevive/otel4s/actions/runs/9029200539/job/24811085216

==> X org.typelevel.otel4s.sdk.trace.SdkTracerSuite.startUnmanaged: use Clock[F].realTime to set start and end if builder's time is undefined  0.133s munit.ComparisonFailException: /home/runner/work/otel4s/otel4s/core/trace/src/test/scala/org/typelevel/otel4s/trace/BaseTracerSuite.scala:649
648:        // _ <- IO.println(tree.map(renderTree).mkString("\n"))
649:      } yield assertEquals(tree, List(expected(now)))
650:    }
values are not the same
=> Obtained
Nil
=> Diff (- obtained, + expected)
-Nil
+List(
+  Impl(
+    current = SpanInfo(
+      name = "span",
+      start = 1000000000 nanoseconds,
+      end = Some(
+        value = 1100000000 nanoseconds
+      )
+    ),
+    children = Nil
+  )
+)