Revision of impala regression tests

[stlemme]

• new cmake option Impala_ENABLE_TESTS (default OFF)
• new library target rtmock, which is the fake runtime (still issues on win32 due to posix)
• intermediate output of tests are placed in build folder not polluting the source tree

$ cd impala/build
$ ctest -C Release -R codegen
64% tests passed, 42 tests failed out of 116

[stlemme]

This addresses #51 but still requires additional work.

• mark broken codegen tests
• freeze output of tests where appropriate
• fix issues in rtmock (linking, required set of functions)
• add support for further test methods (i.e. sema)

[stlemme]

Using the test script without ctest boils down to

cd impala/test
python3 perform.py --libc ../build/lib/librtmock.a codegen/ackermann.impala

if impala as well as clang is in PATH
Then it behaves like the old one and outputs the logs into current working directory.