test: Wait for index sync in wallet_fast_rescan.py #36618

Fixes #36564 by adding the missing wait on the index sync.

Diff to reproduce:

diff --git a/src/util/thread.cpp b/src/util/thread.cpp
index 5b20e807b4..2e988c9eea 100644
--- a/src/util/thread.cpp
+++ b/src/util/thread.cpp
@@ -8,2 +8,3 @@
 #include <util/log.h>
+#include <util/time.h>
 #include <util/threadnames.h>
@@ -17,2 +18,3 @@ void util::TraceThread(std::string_view thread_name, std::function<void()> threa
     util::ThreadRename(std::string{thread_name});
+    UninterruptibleSleep(999ms);
     try {

On failure, the log will say something like:

 - 2026-10-18T05:06:44.451973Z [http.01] [wallet/scan.cpp:142] [ShouldFetchBlock] [scan] Fast rescan: inspect block 200 [4c063a93914ec0cb1604297f2f601dab9b9342bf073e6b80e1c11efd71714e2e] (WARNING: block filter not found!)

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36618
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36618.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36618#author-ralph

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36629 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36629.svg">} (Add output script index and corresponding script history RPC by andrewtoth)
• #36598 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36598.svg">} ([Demo] validation: remove AssumeUTXO by l0rinc)
• #36292 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36292.svg">} (index: Delete old coinstats index directory on startup by fjahr)
• #36257 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36257.svg">} (qa: assert_equals -> assert_true/assert_false by hodlinator)
• #36002 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36002.svg">} (txindex: allow running in pruned mode by andrewtoth)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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It seems a bit tedious to find and fix all of those issues by hand. Also, the tests will become overly verbose with all the manual syncs.

The framework should do this by itself, so I pushed that refactor and removed all of the manual syncs in tests.

Concept ACK, I like how cleaner it is. I also measured performance but saw no significant change.

Commented a test that loses coverage with these changes.