tests: don't silence error reporting for all tests

The test-vmstate test is a bit chatty because it triggers various
expected failure scenarios and the code in question uses error_report
instead of accepting 'Error **errp' parameters. To silence this test the
stubs for error_vprintf() were changed to send errors via
g_test_message() instead of stderr:

  commit 28017e01
  Author: Paolo Bonzini <pbonzini@redhat.com>
  Date:   Mon Oct 24 18:31:03 2016 +0200

    tests: send error_report to test log

    Implement error_vprintf to send the output of error_report to
    the test log.  This silences test-vmstate.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
    Message-Id: <1477326663-67817-3-git-send-email-pbonzini@redhat.com>

Unfortunately this change has global impact across the entire test suite
and means that when tests fail for unexpected reasons, the message is
not displayed on stderr. eg when using &error_abort in a call the test
merely prints

  Unexpected error in qcrypto_tls_session_check_certificate() at crypto/tlssession.c:280:

and the actual error message is hidden, making it impossible to diagnose
the failure. This is especially problematic in CI or build systems where
it isn't possible to easily pass the --debug-log flag to tests and
re-run with the test log visible.

This change makes the previous big hammer much more nuanced, providing a
flag in the stub error_vprintf() that can used on a per-test basis to
silence the errors. Only the test-vmstate silences errors initially.
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

stubs/error-printf.c

@@ -4,7 +4,8 @@
 
 void error_vprintf(const char *fmt, va_list ap)
 {
-    if (g_test_initialized() && !g_test_subprocess()) {
+    if (g_test_initialized() && !g_test_subprocess() &&
+        getenv("QTEST_SILENT_ERRORS")) {
         char *msg = g_strdup_vprintf(fmt, ap);
         g_test_message("%s", msg);
         g_free(msg);

tests/test-vmstate.c

@@ -859,6 +859,8 @@ int main(int argc, char **argv)
 
     module_call_init(MODULE_INIT_QOM);
 
+    setenv("QTEST_SILENT_ERRORS", "1", 1);
+
     g_test_init(&argc, &argv, NULL);
     g_test_add_func("/vmstate/simple/primitive", test_simple_primitive);
     g_test_add_func("/vmstate/versioned/load/v1", test_load_v1);