Merge bitcoin#13533: [tests] Reduced number of validations in tx_validationcache_tests

c3e111a Reduced number of validations in `tx_validationcache_tests` to keep the run time reasonable. (lucash-dev)

Pull request description:

  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.

  Timing for `checkinputs_test`:
  ```
  Before:   6.8s
  After:    3.7s
  ----------------
  Saved:    3.1s (45%)
  ```

  This PR was split from bitcoin#13050. Also see bitcoin#10026.

ACKs for top commit:
  leonardojobim:
    tACK bitcoin@c3e111a.
  kallewoof:
    ACK c3e111a
  theStack:
    re-ACK c3e111a

Tree-SHA512: bef49645bdd4f61ec73cc77a9f028b95d9856db9446d2e7fc9a48867a6f0e94c2c9f150cb771a30fe852db0efb0a1bd15d38b00d712651793ccb59ff6157a7b4

Author: MarcoFalke

src/script/interpreter.h

@@ -101,6 +101,10 @@ enum : uint32_t {
     // Making OP_CODESEPARATOR and FindAndDelete fail
     //
     SCRIPT_VERIFY_CONST_SCRIPTCODE = (1U << 16),
+
+    // 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

@@ -108,10 +108,15 @@ 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, see VerifyScript() in