Mark variables as unused.

clang is smart enough to notice a variable is set but not used,
but these variable are used for testing purposes. Mark one as
unused, and convert the other to a DoNotOptimize.

Bug: 197240255

Test: Compiles without warnings.
Test: Ran unit tests on coral hwasan build.
Change-Id: Ibc873e477355a702579e5a680780c77db0c58046

tests/sys_ttydefaults_test.cpp

@@ -20,7 +20,7 @@
 #include <termios.h>
 
 TEST(sys_ttydefaults, flags) {
-  int i;
+  [[maybe_unused]] int i;
   i = TTYDEF_IFLAG;
   i = TTYDEF_OFLAG;
   i = TTYDEF_LFLAG;

tests/unistd_test.cpp

@@ -688,9 +688,9 @@ __attribute__((noinline)) static void HwasanVforkTestChild() {
 __attribute__((noinline)) static void HwasanReadMemory(const char* p, size_t size) {
   // Read memory byte-by-byte. This will blow up if the pointer tag in p does not match any memory
   // tag in [p, p+size).
-  volatile char z;
+  char z;
   for (size_t i = 0; i < size; ++i) {
-    z = p[i];
+    DoNotOptimize(z = p[i]);
   }
 }