Based on eg 1e6fd9f2b4072873f5d6846b19b2ef0ccc5e4e52, it's understood that
the intention for unix builds is that regular builds disable assert, but
the coverage build should set -O0 and enable asserts.
It looks like this didn't work (even before variants were introduced, eg at
v1.11) -- coverage always built with -Os and -DNDEBUG.
This commit makes it possible for variants to have finer-grained control
over COPT flags, and enables assert() and -O0 on coverage builds.
Other variants already match the defaults so they have been updated.
45cf76465c