Tests: test_filter_flags: Clean up minor issues.

Here are the list of the most significant issues addressed:
- Avoid using internal common.h header. It's not good to copy the
constants like this but common.h cannot be included for use outside
of liblzma. This is the quickest thing to do that could be fixed later.

- Omit the INIT_FILTER macro. Initialization should be done with just
regular designated initializers.

- Use start_offset = 257 for BCJ tests. It demonstrates that Filter
Flags encoder and decoder don't validate the options thoroughly.
257 is valid only for the x86 filter. This is a bit silly but
not a significant problem in practice because the encoder and
decoder initialization functions will catch bad alignment still.
Perhaps this should be fixed but it's not urgent and doesn't need
to be in 5.4.x.

- Various tweaks to comments such as filter id -> Filter ID

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