[libc] reset errno in isatty tests

[michaelrj]

Summary

Errno wasn't getting reset between tests, causing cascading failures.

libc/test/src/unistd/isatty_test.cpp

 #include <errno.h>
 
 using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
 using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
 
 TEST(LlvmLibcIsATTYTest, StdInOutTests) {
   // If stdin is connected to a terminal, assume that all of the standard i/o
   // fds are.
+  errno = 0;
   if (__llvm_libc::isatty(0)) {
     EXPECT_THAT(__llvm_libc::isatty(0), Succeeds(1)); // stdin
     EXPECT_THAT(__llvm_libc::isatty(1), Succeeds(1)); // stdout
     EXPECT_THAT(__llvm_libc::isatty(2), Succeeds(1)); // stderr
   } else {
     EXPECT_THAT(__llvm_libc::isatty(0), Fails(ENOTTY, 0)); // stdin
     EXPECT_THAT(__llvm_libc::isatty(1), Fails(ENOTTY, 0)); // stdout
     EXPECT_THAT(__llvm_libc::isatty(2), Fails(ENOTTY, 0)); // stderr
   }
 }
 
 TEST(LlvmLibcIsATTYTest, BadFdTest) {
+  errno = 0;
   EXPECT_THAT(__llvm_libc::isatty(-1), Fails(EBADF, 0)); // invalid fd
 }
 
 TEST(LlvmLibcIsATTYTest, DevTTYTest) {
   constexpr const char *TTY_FILE = "/dev/tty";
+  errno = 0;
   int fd = __llvm_libc::open(TTY_FILE, O_RDONLY);
   if (fd > 0) {
     ASSERT_EQ(errno, 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";
+  errno = 0;
   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));
 }