[e-acsl] add pathological case for negative integer handling

The -rte flag causes the generation of a negative integer in a rational
context. In that case Translate_terms.constant_to_exp converts the
integer to a real string representation by converting the integer to a
string and then normalising that string using Gmp.Q.normalize_str.

However Gmp.Q.normalize_str does not correctly handle negative integers.

Note that negative integers do not occur when writing for instance -1 in
the source file as it is parsed as TUnOp(Neg, _).

src/plugins/e-acsl/tests/arith/neg_rte_int.c

+/* run.config
+   STDOPT: #"-rte"
+   COMMENT: Generates an RTE with a negative integer
+   COMMENT: (The negative integer being the lower bound of signed chars)
+   COMMENT: This regression test ensures that E-ACSL correctly handles negative integers.
+*/
+char c;
+float f;
+void main() {
+  c = f + 0.5;
+}