[libc] Fix stdio tests after #143802 (#143810)

In #143802 the stdio test cleanup missed a few places where errno was
being set to a failing value, and one where the framework needed to
included.

libc/test/src/stdio/fgetc_test.cpp

@@ -33,6 +33,7 @@ public:
     // This is an error and not a real EOF.
     ASSERT_EQ(LIBC_NAMESPACE::feof(file), 0);
     ASSERT_NE(LIBC_NAMESPACE::ferror(file), 0);
+    ASSERT_ERRNO_FAILURE();
 
     ASSERT_EQ(0, LIBC_NAMESPACE::fclose(file));
 

libc/test/src/stdio/fgetc_unlocked_test.cpp

@@ -36,6 +36,7 @@ public:
     // This is an error and not a real EOF.
     ASSERT_EQ(LIBC_NAMESPACE::feof(file), 0);
     ASSERT_NE(LIBC_NAMESPACE::ferror(file), 0);
+    ASSERT_ERRNO_FAILURE();
 
     ASSERT_EQ(0, LIBC_NAMESPACE::fclose(file));
 

libc/test/src/stdio/fgets_test.cpp

@@ -36,6 +36,7 @@ TEST_F(LlvmLibcFgetsTest, WriteAndReadCharacters) {
   // This is an error and not a real EOF.
   ASSERT_EQ(LIBC_NAMESPACE::feof(file), 0);
   ASSERT_NE(LIBC_NAMESPACE::ferror(file), 0);
+  ASSERT_ERRNO_FAILURE();
 
   ASSERT_EQ(0, LIBC_NAMESPACE::fclose(file));
 

libc/test/src/stdio/setvbuf_test.cpp

@@ -11,6 +11,7 @@
 #include "src/stdio/fread.h"
 #include "src/stdio/fwrite.h"
 #include "src/stdio/setvbuf.h"
+#include "test/UnitTest/ErrnoCheckingTest.h"
 #include "test/UnitTest/Test.h"
 
 #include "hdr/stdio_macros.h"