[wp] refactor prover selection and cleanup tests

src/plugins/wp/VCS.ml

src/plugins/wp/Why3Provers.ml

src/plugins/wp/Why3Provers.mli

src/plugins/wp/gui/GuiConfig.ml

src/plugins/wp/tests/wp_acsl/chunk_typing_usable.i

src/plugins/wp/tests/wp_acsl/classify_float.c

src/plugins/wp/tests/wp_acsl/div_mod.i

src/plugins/wp/tests/wp_acsl/float_compare.i

...p/tests/wp_acsl/oracle_qualif/classify_float.2.res.oracle

...ugins/wp/tests/wp_acsl/oracle_qualif/div_mod.1.res.oracle

...ugins/wp/tests/wp_acsl/oracle_qualif/div_mod.2.res.oracle

...plugins/wp/tests/wp_acsl/oracle_qualif/div_mod.res.oracle

src/plugins/wp/tests/wp_acsl/tset.i

src/plugins/wp/tests/wp_gallery/binary-multiplication.c

src/plugins/wp/tests/wp_plugin/abs.i

src/plugins/wp/tests/wp_plugin/bit_test.c

src/plugins/wp/tests/wp_plugin/fallback.i

src/plugins/wp/tests/wp_plugin/float_format.i

src/plugins/wp/tests/wp_plugin/math.i

src/plugins/wp/tests/wp_plugin/nosession.i