Fix mempool_tests.cpp Unit Test

[JaredTate]

This PR begins work to fix the Unit tests. Unit tests are the most important C++ tests located in the src folder and are what first runs when you make check. This fixes mempool_tests.cpp & miner_tests.cpp. I don't know how many of these are failing but will take them one by one.

You can read more about Unit Tests here:
https://github.com/DigiByte-Core/digibyte/blob/develop/src/test/README.md

To Compile & Run Unit Tests:

  ./autogen.sh
  ./configure
  make -j 8
  make check

Run Specific Unit Test Fixed

cd src/test
./test_digibyte --run_test=mempool_tests --log_level=message
./test_digibyte --run_test=miner_tests --log_level=message

You can also re-run make check but it will just take longer for a specific test.

It is important to note while trying to fix these Unit tests you need to make every time you make a change to recompile & re-run test.

I am using --log_level=message and BOOST_TEST_MESSAGE to help me trouble shoot these unit tests with log outputs.

[Jongjan88]

ACK Tested nice work @JaredTate

[gto90]

ACK

[ycagel]

cACK. Great work @JaredTate!