Replace .github/test.sh with a unified set of targets installed
via `make check-*` in ivtest/, thereby removing CI-specific test
coordination. This avoids duplication in the regression logic and
ensures consistent execution between local and CI environments.
PLI1-dependent tests are now correctly controlled via
`configure --enable-libveriuser`.
Currently, the regression suite still depends on an iverilog package,
which must be installed manually at the location specified with
`configure --prefix=*`. Afterward, the complete regression suite
(VVP, VPI, and Python tests) can be run via `make check-installed`
and individual checks can be run with `check-installed-vpi`,
`check-installed-vvp` and `check-installed-vvp-py`.
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