[libc] handle case where /dev/tty doesn't exist

[michaelrj]

Summary

In the isatty test it was assumed that /dev/tty existed, but that's not
always the case. Now we check for that.

libc/test/src/unistd/isatty_test.cpp

 
 TEST(LlvmLibcIsATTYTest, BadFdTest) {
   EXPECT_THAT(__llvm_libc::isatty(-1), Fails(EBADF, 0)); // invalid fd
 }
 
 TEST(LlvmLibcIsATTYTest, DevTTYTest) {
   constexpr const char *TTY_FILE = "/dev/tty";
   int fd = __llvm_libc::open(TTY_FILE, O_RDONLY);
+  if (fd > 0) {
     ASSERT_EQ(errno, 0);
-  ASSERT_GT(fd, 0);
     EXPECT_THAT(__llvm_libc::isatty(fd), Succeeds(1));
     ASSERT_THAT(__llvm_libc::close(fd), Succeeds(0));
   }
+}
 
 TEST(LlvmLibcIsATTYTest, FileTest) {
   constexpr const char *TEST_FILE = "testdata/isatty.test";
   int fd = __llvm_libc::open(TEST_FILE, O_WRONLY | O_CREAT, S_IRWXU);
   ASSERT_EQ(errno, 0);
   ASSERT_GT(fd, 0);
   EXPECT_THAT(__llvm_libc::isatty(fd), Fails(ENOTTY, 0));
   ASSERT_THAT(__llvm_libc::close(fd), Succeeds(0));
 }