Add txrequest unit tests

Add unit tests for TxRequestTracker. Several scenarios are tested,
randomly interleaved with eachother.

Includes a test by Antoine Riard (ariard).

Author: sipa

src/Makefile.test.include

@@ -275,6 +275,7 @@ BITCOIN_TESTS =\
   test/torcontrol_tests.cpp \
   test/transaction_tests.cpp \
   test/txindex_tests.cpp \
+  test/txrequest_tests.cpp \
   test/txvalidation_tests.cpp \
   test/txvalidationcache_tests.cpp \
   test/uint256_tests.cpp \