libc: fix setvbuf_fullbuffer_overflow testcase

The test case checks fully buffered mode.
Previous implementation required data overflowing the buffer
(sending 10 bytes when a buffer is 8 bytes long) to be written to a file at once
which is not required by POSIX.
The current solution forces flushing the remaining 2 bytes anyway.

JIRA: CI-507

libc/stdio/stdio_file.c

@@ -980,7 +980,10 @@ TEST(stdio_bufs, setvbuf_fullbuffer_overflow)
 	TEST_ASSERT_EQUAL_INT(0, setvbuf(filep, buf2, _IOFBF, sizeof(buf2)));
 
 	TEST_ASSERT_GREATER_THAN_INT(0, fputs(data, filep));
+	/* Flush is used, because it's possible that overflow data will be written anyway (glibc) */
+	fflush(filep);
 	TEST_ASSERT_NOT_NULL(fgets(buf, sizeof(buf), filep2));
+	TEST_ASSERT_EQUAL_STRING(data, buf);
 	TEST_ASSERT_EQUAL_INT(strlen(data), strlen(buf));
 }