Fix `--test` behavior for gromacs package. (#35674)

For `spack install --test=all gromacs`
* remove the `test` target from the `check()` call and just use
  the `check` target, in accordance with usual GROMACS test protocol
* build the test binaries explicitly during the build phase

Additional minor updates are necessary. This change
updates the package structure to the newer format with a
separate Builder class so we can override `check()`.
However, note that additional modernization should be
undertaken with care.

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