Add regression tests for UNREACHABLE_BECAUSE

Author: Enrico Steffinlongo

regression/invariants/driver.cpp

@@ -112,6 +112,8 @@ int main(int argc, char** argv)
     UNREACHABLE_STRUCTURED(structured_error_testt, 1, "Structured error"); // NOLINT
   else if(arg=="unreachable-string")
     UNREACHABLE;
+  else if(arg == "unreachable-because")
+    UNREACHABLE_BECAUSE("Unreachable with explanation text");
   else if(arg=="data-invariant-structured")
     DATA_INVARIANT_STRUCTURED(false, structured_error_testt, 1, "Structured error"); // NOLINT
   else if(arg=="data-invariant-string")

regression/invariants/invariant-failure21/test.desc

@@ -0,0 +1,12 @@
+CORE
+dummy_parameter.c
+unreachable-because
+^EXIT=(0|127|134|137)$
+^SIGNAL=0$
+--- begin invariant violation report ---
+Unreachable with explanation text
+Invariant check failed
+^(Backtrace)|(Backtraces not supported)$
+--
+^warning: ignoring
+^VERIFICATION SUCCESSFUL$