test: skip test if any of the needed release binaries is missing

If the `releases` directory exists, but still only a subset of the
necessary previous release binaries are available, the test fails by
throwing an exception (sometimes leading to follow-up exceptions like
"AssertionError: [node 0] Error: no RPC connection") and printing out
a stack trace, which can be confusing and at a first glance suggests
that the node crashed or some alike.
Improve this by checking and printing out *all* of the missing release
binaries and skip the test in this case. Noticed while testing bitcoin#30328.

Can be tested by e.g.

$ ./test/get_previous_releases -b
$ rm -rf ./releases/v28.0/
$ ./build/test/functional/wallet_migration.py

test/functional/test_framework/test_framework.py

@@ -526,6 +526,14 @@ def get_bin_from_version(version, bin_name, bin_default):
             binary = [get_bin_from_version(v, 'bitcoind', self.options.bitcoind) for v in versions]
         if binary_cli is None:
             binary_cli = [get_bin_from_version(v, 'bitcoin-cli', self.options.bitcoincli) for v in versions]
+        # Skip test if any of the needed release binaries is missing
+        bins_missing = False
+        for bin_path in binary + binary_cli:
+            if not os.path.exists(bin_path):
+                self.log.error(f"Binary not found: {bin_path}")
+                bins_missing = True
+        if bins_missing:
+            raise SkipTest("At least one release binary is missing")
         assert_equal(len(extra_confs), num_nodes)
         assert_equal(len(extra_args), num_nodes)
         assert_equal(len(versions), num_nodes)