test: Print os exit code on failure #36010

Printing the exit code (like printing the stderr) seems independently useful, but should also help to debug the Windows CI failures, which have an empty stderr and truncated combined log:

• #34925
• #34367
• ...

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36010.

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Can be tested by applying this diff:

diff --git a/test/functional/wallet_disable.py b/test/functional/wallet_disable.py
index 17b327e17e..781a7e51a6 100755
--- a/test/functional/wallet_disable.py
+++ b/test/functional/wallet_disable.py
@@ -20,3 +20,6 @@ class DisableWalletTest (BitcoinTestFramework):
 
-    def run_test (self):
+    def run_test(self):
+        self.stop_nodes()
+        import os
+        os._exit(123)
         # Make sure wallet is really disabled

And then running it. E.g:

./bld-cmake/test/functional/test_runner.py wallet_disable --combinedlogslen 5

tACK fada80192bdd9e350424591661fe418029c8e64e