Fix exit code in Formal Verification results.py (#2539)

Currently formal verification script `results.py` is only returning
value that should be exit code, but it always exits with 0.
This PR fixes this.

We also noticed some issue with Surelog cache in formal verification, so
as a workaround, we are disabling it temporarily.

tests/formal/results.py

@@ -132,4 +132,4 @@ def main():
     return 0
 
 if __name__ == "__main__":
-    main()
+    exit(main())

tests/formal/run.py

@@ -63,7 +63,7 @@ def prepare_eqy_script(output_dir, script_name, plugin_file, yosys_file):
         "",
         "[gate]",
         "plugin -i systemverilog",
-        "tee -o %s/%s/plugin_ast.txt read_systemverilog -debug %s" % (output_dir, script_name, plugin_file),
+        "tee -o %s/%s/plugin_ast.txt read_systemverilog -nocache -debug %s" % (output_dir, script_name, plugin_file),
         "prep -flatten -auto-top",
         "opt",
         "",