Add clang_parser::parse_fail test

Adding clang_parser::parse_fail test to test the
failure of the ClangParser::parse.

Testing the standard 'undefined struct' failure
for folowing struct:

  struct a {
    int a;
    struct b b;
  };

Also adding check on ClangParser::parse return value
in all the clang_parser tests.

tests/clang_parser.cpp

@@ -7,14 +7,14 @@ namespace bpftrace {
 namespace test {
 namespace clang_parser {
 
-void parse(const std::string &input, BPFtrace &bpftrace)
+static void parse(const std::string &input, BPFtrace &bpftrace, int result = 0)
 {
   auto extended_input = input + "kprobe:sys_read { 1 }";
   Driver driver(bpftrace);
   ASSERT_EQ(driver.parse_str(extended_input), 0);
 
   ClangParser clang;
-  clang.parse(driver.root_, bpftrace);
+  ASSERT_EQ(clang.parse(driver.root_, bpftrace), result);
 }
 
 TEST(clang_parser, integers)
@@ -284,6 +284,12 @@ TEST(clang_parser, macro_preprocessor)
   ASSERT_EQ(macros["FOO"], "size_t");
 }
 
+TEST(clang_parser, parse_fail)
+{
+  BPFtrace bpftrace;
+  parse("struct a { int a; struct b b; };", bpftrace, 1);
+}
+
 } // namespace clang_parser
 } // namespace test
 } // namespace bpftrace