Add test for u8 align_offset

tests/run-pass/aligned_utf8_check.rs

@@ -0,0 +1,13 @@
+fn main() {
+    const N: usize = 10;
+
+    let x = vec![0x4141u16; N];
+
+    let mut y: Vec<u8> = unsafe { std::mem::transmute(x) };
+    unsafe { y.set_len(2 * N) };
+
+    println!("{:?}", String::from_utf8_lossy(&y));
+
+    let mut x: Vec<u16> = unsafe { std::mem::transmute(y) };
+    unsafe { x.set_len(N) };
+}

tests/run-pass/aligned_utf8_check.stdout

@@ -0,0 +1 @@
+"AAAAAAAAAAAAAAAAAAAA"