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 Job ID Name Coverage
  Build
passed #42372
alpine-gcc

00:26:27

passed #42373
arch-clang

00:46:18

passed #42374
arch-gcc-glibcxxdebug

00:44:30

passed #42371
debian-gcc-snapshot

01:15:00

passed #42369
debian-stable-gcc

00:39:33

passed #42370
debian-unstable-gcc-coverage

00:39:47

88.8%
passed #42377
debpkg-stable

01:59:07

passed #42375
mingw-shared

00:10:03

passed #42376
mingw-static

00:07:44

passed #42378
rpm-pkg

01:01:47