Add test for attribute signal

Just to check that gnat2goto will not produce the unsupported feature report.

Author: Petr Bauch

testsuite/gnat2goto/tests/pragma_machine_attribute/pragma_machine_attribute.adb

@@ -0,0 +1,23 @@
+procedure Pragma_Machine_Attribute is
+
+   procedure Timer_Interrupt;
+   pragma Machine_Attribute (Timer_Interrupt, "signal");
+
+   Seconds_Count : Integer := 0;
+   Interval : Integer := 1;
+   Timeout : Boolean := False;
+
+   procedure Timer_Interrupt is
+   begin
+      Seconds_Count := Seconds_Count + 1;
+      if Seconds_Count = Interval then
+         Timeout := True;
+      end if;
+   end Timer_Interrupt;
+
+begin
+   Timer_Interrupt;
+
+   pragma Assert (Timeout and Interval = Seconds_Count);
+
+end Pragma_Machine_Attribute;

testsuite/gnat2goto/tests/pragma_machine_attribute/test.out

@@ -0,0 +1,2 @@
+[1] file pragma_machine_attribute.adb line 21 assertion: SUCCESS
+VERIFICATION SUCCESSFUL

testsuite/gnat2goto/tests/pragma_machine_attribute/test.py

@@ -0,0 +1,3 @@
+from test_support import *
+
+prove()