Suppress -Wformat warning

This is exactly what it is testing for.

Original warning:
  bionic/tests/stdio_test.cpp:370:47: error: '%n' specifier not supported on this platform [-Werror,-Wformat]
    EXPECT_DEATH(snprintf(buf, sizeof(buf), "a %n b", &i), "%n not allowed on Android");
                                               ~^

Test: presubmit
Bug: 219872355
Change-Id: I6e378722b2d681cf64f4cf31ef000bd28203b00d
diff --git a/tests/stdio_test.cpp b/tests/stdio_test.cpp
index e38dd60..87031f6 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -364,10 +364,13 @@
 
 TEST_F(STDIO_DEATHTEST, snprintf_n) {
 #if defined(__BIONIC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat"
   // http://b/14492135 and http://b/31832608.
   char buf[32];
   int i = 1234;
   EXPECT_DEATH(snprintf(buf, sizeof(buf), "a %n b", &i), "%n not allowed on Android");
+#pragma GCC diagnostic pop
 #else
   GTEST_SKIP() << "glibc does allow %n";
 #endif