Adapt tests due to the new Clang fortify-source warning

https://reviews.llvm.org/D58797 added several new compile time
_FORTIFY_SOURCE diagnostics. This broke clang_fortify_tests:

FAILED: out/target/product/walleye/obj/STATIC_LIBRARIES/bionic-compile-time-tests2-clang++_intermediates/clang_fortify_tests.o
error: 'warning' diagnostics seen but not expected:
  Line 159: 'memcpy' will always overflow; destination buffer has size 8, but size argument is 9
  Line 161: 'memmove' will always overflow; destination buffer has size 8, but size argument is 9
  Line 165: 'memset' will always overflow; destination buffer has size 8, but size argument is 9
  Line 183: 'strncpy' size argument is too large; destination buffer has size 8, but size argument is 9
  Line 185: 'stpncpy' size argument is too large; destination buffer has size 8, but size argument is 9
  Line 189: 'strncat' size argument is too large; destination buffer has size 8, but size argument is 9
  Line 227: 'strncpy' size argument is too large; destination buffer has size 4, but size argument is 5
  Line 232: 'stpncpy' size argument is too large; destination buffer has size 4, but size argument is 5
  Line 242: 'strncat' size argument is too large; destination buffer has size 4, but size argument is 5
  Line 490: 'snprintf' size argument is too large; destination buffer has size 8, but size argument is 9
  Line 495: 'vsnprintf' size argument is too large; destination buffer has size 8, but size argument is 9
error: 'note' diagnostics seen but not expected:
  File bionic/libc/include/bits/fortify/fcntl.h Line 50: 'open' has been explicitly marked unavailable here
  File bionic/libc/include/bits/fortify/fcntl.h Line 102: 'open64' has been explicitly marked unavailable here
  File bionic/libc/include/bits/fortify/fcntl.h Line 75: 'openat' has been explicitly marked unavailable here
  File bionic/libc/include/bits/fortify/fcntl.h Line 121: 'openat64' has been explicitly marked unavailable here
  File bionic/libc/include/bits/fortify/stdio.h Line 68: 'sprintf' has been explicitly marked unavailable here
16 errors generated.

Bug: 131328001
Bug: 138701853
Test: Build with r365631 toolchain
Change-Id: I103ecb7b3abcf4b5cfc4f36ccab69e03d0976275
diff --git a/tests/clang_fortify_tests.cpp b/tests/clang_fortify_tests.cpp
index 1b6b898..6923302 100644
--- a/tests/clang_fortify_tests.cpp
+++ b/tests/clang_fortify_tests.cpp
@@ -36,6 +36,11 @@
 // Similarly, there are a few overload tricks we have to emit errors. Ignore any notes from those.
 // expected-note@* 0+{{candidate function}}
 
+// FIXME(b/138701943): Silence warnings produced by -Wfortify-source since they're expected.
+// expected-warning@* 0+{{will always overflow}}
+// expected-warning@* 0+{{size argument is too large}}
+// expected-note@* 0+{{has been explicitly marked unavailable here}}
+
 #ifndef _FORTIFY_SOURCE
 #error "_FORTIFY_SOURCE must be defined"
 #endif