test: descriptor: bare multisig at TOP level with exactly 3 pubkeys is allowed #35540

When running mutation testing for src/script/descriptor.cpp, I noticed that the following mutant was not killed:

diff --git a/src/script/descriptor.cpp b/src/script/descriptor.cpp
index 3b73a40cce..29775ba75f 100644
--- a/src/script/descriptor.cpp
+++ b/src/script/descriptor.cpp
@@ -2393,7 +2393,7 @@ std::vector<std::unique_ptr<DescriptorImpl>> ParseScript(uint32_t& key_exp_index
             return {};
         }
         if (ctx == ParseScriptContext::TOP) {
-            if (providers.size() > 3) {
+            if (providers.size() >= 3) {
                 error = strprintf("Cannot have %u pubkeys in bare multisig; only at most 3 pubkeys", providers.size());
                 return {};
             }

We current have a test case that verifies we cannot have 4 pubkeys in bare multisig, but we are not testing that exactly 3 pubkeys are allowed. This PR adds a test case for it.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35540.

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