tests: Conditional compilation of tests (including boost-based unit tests)

To compile tests, code needs to be configured with --with-tests option,
otherwise all tests are disabled.

Tests that are based on Boost's unit test framework are enabled only if
the framework is detected during the configure time.

Change-Id: Ia1768e24b088e4f19d2a8799c6d5baeabb2f44c5
diff --git a/.gitignore b/.gitignore
index 7c6af0e..ae3de42 100644
--- a/.gitignore
+++ b/.gitignore
@@ -68,3 +68,7 @@
 tests/test-encode-decode-interest
 tests/test-get-async
 tests/test-publish-async
+
+# Tests using Boost's unit test framework
+tests_boost/unit-tests
+