Build the main c-ares library first, in the directory above this. To enable tests of internal functions, configure the library build to expose hidden symbols with ./configure --disable-symbol-hiding.
Generate a configure file by running autoreconf -iv (which requires a local installation of autotools).
./configure
make
Run the tests with ./arestest, or ./arestest -v for extra debug info.

Points to note:

The tests are written in C++11, and so need a C++ compiler that supports this. To avoid adding this as a requirement for the library, the configuration and build of the tests is independent from the library.
The tests include some live queries, which will fail when run on a machine without internet connectivity. To skip live tests, run with ./arestest --gtest_filter=-*.Live*.
The tests include queries of a mock DNS server. This server listens on port 5300 by default, but the port can be changed with the -p 5300 option to arestest.

Test Types

The test suite includes various different types of test.

There are live tests (ares-test-live.cc), which assume that the current machine has a valid DNS setup and connection to the internet; these tests issue queries for real domains but don't particularly check what gets returned. The tests will fail on an offline machine.
There are some mock tests (ares-test-mock.cc) that set up a fake DNS server and inject its port into the c-ares library configuration. These tests allow specific response messages to be crafted and injected, and so are likely to be used for many more tests in future.
- To make this generation/injection easier, the dns-proto.h file includes C++ helper classes for building DNS packets.
Other library entrypoints that don't require network activity (e.g. ares_parse_*_reply) are tested directly.
A couple of the tests use a helper method of the test fixture to inject memory allocation failures, using a recent change to the c-ares library that allows override of malloc/free.
There are some tests of the internal entrypoints of the library (ares-test-internal.c), but these are only enabled if the library was configured with --disable-symbol-hiding and/or --enable-expose-statics.
There is also an entrypoint to allow Clang's libfuzzer to drive the packet parsing code in ares_parse_*_reply, together with a standalone wrapper for it (./aresfuzz) to allow use of command line fuzzers (such as afl-fuzz) for further fuzz testing.

Code Coverage Information

To generate code coverage information:

Configure both the library and the tests with ./configure --enable-code-coverage before building. This requires the relevant code coverage tools (gcov, lcov) to be installed locally.
Run the tests with test/arestest.
Generate code coverage output with make code-coverage-capture in the library directory (i.e. not in test/).