test: correct main_tests (subsidy)

[SmartArray]

DigiByte specific main_tests (subsidy)

What is the intention of this PR?

This PR fixes the main_tests with DigiByte's specific Supply Curve Parameters.
Unfortunately, there have been many hard forks along the road of DigiByte, and almost all of them seem to have altered the supply parameters, which basically means we can't just loop through the halvings and validate the supply curve.

Be warned: This unit test was adapted to check the whole supply curve, even if the supply curve itself is erroneous. A new PR which alters the supply curve parameters must also provide an updated main_tests unit test.
This test will take some time to run due to the huge number of blocks it iterates through (6-8 min). The new code, in which the supply curve parameters have been altered, needs 6 different loops to test each and every hard fork (starting here: https://github.com/SmartArray/digibyte/blob/1dde1ec334a33e756a0724a3a0c679b16143bc80/src/test/main_tests.cpp#L62)

Description of the changes

The commit SmartArray@5747bb3 updated the main_tests unit test in order to test DigiBytes Subsidy.

SmartArray@1dde1ec adds a major algorithmical optimization in order to drastically reduce the runtime of this unit test.

How to verify?

1. Change to the directory src/test
2. Compile the test suite
3. Run it using

./test_digibyte --run_test=main_tests

Expected outcome

$ ./test_digibyte --run_test=main_tests
Running 2 test cases...
*** No errors detected

Remarks

This doesn't fix the issue discussed in #20

[gto90]

utACK

[digicontributer]

ACK