formula: b* is siSERE

Since b[+] and [*0] are siSERE, b* is siSERE as well.
Suggested by Victor Khomenko.

* spot/tl/formula.cc: Implement that for Star and also
in the concatenation rule.
* tests/core/kind.test, tests/core/ltlfilt.test: Adjust.
10 jobs for master in 147 minutes and 57 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed alpine-gcc #42372

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passed arch-clang #42373

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passed arch-gcc-glibcxxdebug #42374

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passed debian-gcc-snapshot #42371

01:15:00

passed debian-stable-gcc #42369

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passed debian-unstable-gcc-coverage #42370

00:39:47

88.8%
passed debpkg-stable #42377

01:59:07

passed mingw-shared #42375

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passed mingw-static #42376

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passed rpm-pkg #42378

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