Add unit tests for next difficulty calculations

Split GetNextWorkRequired() into two functions to allow the difficulty calculations to
be tested without requiring a full blockchain.
Add unit tests to cover basic difficulty calculation, plus each of the min/max actual
time, and maximum difficulty target conditions.

src/Makefile.test.include

@@ -55,6 +55,7 @@ BITCOIN_TESTS =\
   test/multisig_tests.cpp \
   test/netbase_tests.cpp \
   test/pmt_tests.cpp \
+  test/pow_tests.cpp \
   test/rpc_tests.cpp \
   test/sanity_tests.cpp \
   test/script_P2SH_tests.cpp \