Reduced number of validations in tx_validationcache_tests to keep the run time reasonable.

Following a suggestion in the comments, changed `ValidateCheckInputsForAllFlags` from testing all possible flag combinations to testing a random subset. Also created a new enum constant for the highest flag, so that this test doesn’t keep testing an incomplete subset in case a new flag is added.

src/script/interpreter.h

@@ -139,6 +139,10 @@ enum
 
     // Making unknown public key versions (in BIP 342 scripts) non-standard
     SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_PUBKEYTYPE = (1U << 20),
+
+    // Constants to point to the highest flag in use. Add new flags above this line.
+    //
+    SCRIPT_VERIFY_END_MARKER
 };
 
 bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned int flags, ScriptError* serror);

src/test/txvalidationcache_tests.cpp

@@ -109,10 +109,15 @@ BOOST_FIXTURE_TEST_CASE(tx_mempool_block_doublespend, TestChain100Setup)
 static void ValidateCheckInputsForAllFlags(const CTransaction &tx, uint32_t failing_flags, bool add_to_cache) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     PrecomputedTransactionData txdata;
-    // If we add many more flags, this loop can get too expensive, but we can
-    // rewrite in the future to randomly pick a set of flags to evaluate.
-    for (uint32_t test_flags=0; test_flags < (1U << 16); test_flags += 1) {
+
+    FastRandomContext insecure_rand(true);
+
+    for (int count = 0; count < 10000; ++count) {
         TxValidationState state;
+
+        // Randomly selects flag combinations
+        uint32_t test_flags = (uint32_t) insecure_rand.randrange((SCRIPT_VERIFY_END_MARKER - 1) << 1);
+
         // Filter out incompatible flag choices
         if ((test_flags & SCRIPT_VERIFY_CLEANSTACK)) {
             // CLEANSTACK requires P2SH and WITNESS, see VerifyScript() in