build: avoid clang's colored diagnostics in the test suite

The syncline tests, which try to recognize compiler diagnostics,
are confused by escapes for colors.

* configure.ac (warn_tests): New, to factor the warnings for both
C and C++ tests.
Add -fno-color-diagnostics to it.
* tests/local.at (AT_TEST_TABLES_AND_PARSE): Do not remove glue
together compiler flags.