D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082
A 'test_args.txt' file in the same directory as the test case will be
parsed by the test framework and the content will be passed as arguments
to meson during configuration. The arguments are put before any
'extra_args' to make them overwritable from the command line.