add two tests for error reporting when using --json-ui

Error-reporting is important, yet undertested, as shown by #3897.

Author: Daniel Kroening

regression/cbmc/json-ui/no_entry.c

@@ -0,0 +1,4 @@
+int I_am_not_main()
+{
+  return 0;
+}

regression/cbmc/json-ui/no_entry.desc

@@ -0,0 +1,10 @@
+KNOWNBUG
+no_entry.c
+--json-ui
+activate-multi-line-match
+^EXIT=6$
+^SIGNAL=0$
+"messageText": "the program has no entry point",[\n ]*"messageType": "ERROR",
+--
+^warning: ignoring
+^VERIFICATION SUCCESSFUL$

regression/cbmc/json-ui/syntax_error.c

@@ -0,0 +1,6 @@
+int main()
+{
+  // clang-format off
+  I AM A SYNTAX ERROR
+  return 0;
+}

regression/cbmc/json-ui/syntax_error.desc

@@ -0,0 +1,10 @@
+KNOWNBUG
+syntax_error.c
+--json-ui
+activate-multi-line-match
+^EXIT=6$
+^SIGNAL=0$
+"messageText": "syntax error before .*",[\n ]*"messageType": "ERROR",[\n ]*"sourceLocation": {[\n ]*"file": "syntax_error.c",[\n ]*"function": "main",[\n ]*"line": "4",
+--
+^warning: ignoring
+^VERIFICATION SUCCESSFUL$