negation + better tests

src/plugins/e-acsl/tests/e-acsl-runtime/empty.i

src/plugins/e-acsl/tests/e-acsl-runtime/false.i

src/plugins/e-acsl/tests/e-acsl-runtime/false.i~

src/plugins/e-acsl/tests/e-acsl-runtime/not.i

...ugins/e-acsl/tests/e-acsl-runtime/oracle/false.err.oracle

...ugins/e-acsl/tests/e-acsl-runtime/oracle/false.res.oracle

...plugins/e-acsl/tests/e-acsl-runtime/oracle/not.err.oracle

...plugins/e-acsl/tests/e-acsl-runtime/oracle/not.res.oracle

...lugins/e-acsl/tests/e-acsl-runtime/oracle/true.err.oracle

...lugins/e-acsl/tests/e-acsl-runtime/oracle/true.res.oracle

src/plugins/e-acsl/tests/e-acsl-runtime/true.i

src/plugins/e-acsl/visit.ml