Added testcase for ignore() unsafety

[mitsuhiko]

I noticed that there were miri tests and I was curious why #24 was not failing. I managed to reproduce this by actually trying to make a test that uses ignore. The test is failing locally.

test regression::issue24::main ... error: Undefined Behavior: pointer to alloc63740 was dereferenced after this allocation got freed
  --> /Users/mitsuhiko/Development/miniserde/src/ignore.rs:7:18
   |
7  |         unsafe { extend_lifetime!(&mut Ignore as &mut Ignore) }
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ pointer to alloc63740 was dereferenced after this allocation got freed
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

[dtolnay]

Thanks!