Add a new CMake target "tests"

This commit adds a new custom target called "tests". In contrast with
the standard "test" target it has all the test binaries listed as its
dependencies. Consequently, a single command `make tests` is sufficient
to build all tests (and nothing else) and run them.
Enable verbose output for "tests" target

This commit adds "-V" switch to the invocation of `ctest` in the "tests"
target. Doing so allows us to see the output of individual tests in
addition to the summary provided by `ctest`.
Merge pull request #420 from taketwo/add-tests-target

Add a new CMake target "tests"