void test_modifyTimestamp(int _timestamp)

As some transactions execute TIMESTAMP opcode it is needed to set the timestamp of the current block that is being mined with that transaction so that a transaction could calculate the desired TIMESTAMP value;

The RPC call sequence is following:

session.test_modifyTimestamp(a.convert_to<size_t>());
session.test_addTransaction(tr.transaction.getData().asJson());
session.test_mineBlocks(1);

First set the timestamp of the current block. Then calculate some transaction with that timestamp.
Then finish and calculate the post state hash of a new block.

*This is basically needed to check that TIMESTAMP related tests has a determined results. But is might be not a good solution so far. A disscussion is welcome.