lib-test: test-exit helper to stop valgrind complaining about fork()s

fork()s that want to _exit() will cause valgrind's full memory leak
checker to complain a lot - this performs a quick cleanup first.

Before:
  phil@phil:~/repos/dovecot-core$ valgrind --trace-children=yes --leak-check=full --show-leak-kinds=all src/lib/test-lib --match unix
  ==19576==   total heap usage: 4 allocs, 0 frees, 17,858 bytes allocated
  istream unix ......................................................... : ok
  0 / 1 tests failed
  ==19575==   total heap usage: 7 allocs, 7 frees, 19,327 bytes allocated

After:
  phil@phil:~/repos/dovecot-core$ valgrind --trace-children=yes --leak-check=full --show-leak-kinds=all src/lib/test-lib --match unix
  istream unix ......................................................... : ok
  ==4993==   total heap usage: 4 allocs, 4 frees, 17,858 bytes allocated
  0 / 1 tests failed
  ==4992==   total heap usage: 7 allocs, 7 frees, 19,327 bytes allocated

Signed-off-by: Phil Carmody <phil@dovecot.fi>

src/lib-test/test-common.c

@@ -5,7 +5,7 @@
 #include "test-common.h"
 
 #include <stdio.h>
-
+#include <unistd.h> /* _exit() */
 #include <setjmp.h> /* for fatal tests */
 
 /* To test the firing of i_assert, we need non-local jumps, i.e. setjmp */
@@ -448,3 +448,13 @@ int test_run_named_with_fatals(const char *match, struct named_test tests[],
 	test_run_named_fatals(fatals, match);
 	return test_deinit();
 }
+
+void ATTR_NORETURN
+test_exit(int status)
+{
+	i_free_and_null(expected_error_str);
+	i_free_and_null(test_prefix);
+	(void)t_pop(); /* as we were within a T_BEGIN { tests[i].func(); } T_END */
+	lib_deinit();
+	_exit(status);
+}

src/lib-test/test-common.h

@@ -73,4 +73,7 @@ int test_run_named_with_fatals(const char *match, struct named_test tests[],
 #define FATAL_NAMELESS(x) x, /* Were you to want to use the X trick but not name the tests */
 #define FATAL_NAMED(x) { .name = #x , .func = x },
 
+/* If a fork() wants to exit(), then this will avoid valgrind leak errors */
+void test_exit(int status) ATTR_NORETURN;
+
 #endif

src/lib/test-istream-unix.c

@@ -176,7 +176,7 @@ void test_istream_unix(void)
 	case 0:
 		i_close_fd(&fd[0]);
 		test_istream_unix_client(fd[1]);
-		_exit(0);
+		test_exit(0);
 	default:
 		i_close_fd(&fd[1]);
 		test_istream_unix_server(fd[0]);