[libc] reset errno in isatty tests

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

Differential Revision: https://reviews.llvm.org/D135627

libc/test/src/unistd/isatty_test.cpp

@@ -20,6 +20,7 @@ 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
@@ -32,11 +33,13 @@ TEST(LlvmLibcIsATTYTest, StdInOutTests) {
 }
 
 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);
@@ -47,6 +50,7 @@ TEST(LlvmLibcIsATTYTest, DevTTYTest) {
 
 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);