autest: change the way we build the *.c and *.cc files

[bryancall]

We are not building the C and C++ code under the tests directory on all of our supported platforms and we are not running them through clang-analyzer.

They should build as part of the build system, with a configure flag. I would recommend renaming --enable-test-tools (doesn't look like it is used) and rename it to --enable-tests.

[ywkaras]

Here is the point in the python code where Au tests build C/C++ into test-specific plugins:

trafficserver/tests/gold_tests/autest-site/trafficserver_plugins.test.ext

Line 43 in 0f1f028

# Compile the plugin.

It uses tsxs.

It would be hypothetically possible for an Au test to generate C/C++ code when it ran, then build a plugin from it. But I don't think any test does this currently. We could move all plugins for core Au tests to the tree under the core plugins subdirectory.