More robust tests for builtins

src/plugins/e-acsl/share/e-acsl/e_acsl_assert.h

src/plugins/e-acsl/tests/builtin/oracle/gen_strcat.c

src/plugins/e-acsl/tests/builtin/oracle/gen_strcmp.c

src/plugins/e-acsl/tests/builtin/oracle/gen_strcpy.c

src/plugins/e-acsl/tests/builtin/oracle/gen_strlen.c

src/plugins/e-acsl/tests/builtin/oracle/strcat.res.oracle

src/plugins/e-acsl/tests/builtin/oracle/strcmp.res.oracle

src/plugins/e-acsl/tests/builtin/oracle/strcpy.res.oracle

src/plugins/e-acsl/tests/builtin/oracle/strlen.res.oracle

src/plugins/e-acsl/tests/builtin/signalled.c

src/plugins/e-acsl/tests/builtin/signalled.h

src/plugins/e-acsl/tests/builtin/strcat.c

src/plugins/e-acsl/tests/builtin/strcmp.c

src/plugins/e-acsl/tests/builtin/strcpy.c

src/plugins/e-acsl/tests/builtin/test_config