A more verbose matching failure for mir tests

[bluss]

This makes it easier to work with mir test failures during development.

• Show which expected line was not found
• Show full expected output
• Show full actual output

[rust-highfive]

r? @brson

(rust_highfive has picked a reviewer for you, use r? to override)

[bluss]

Example output below. It's long, but it's useful when developing.

failures: ---- [mir-opt] mir-opt/deaggregator_test.rs stdout ----
thread '[mir-opt] mir-opt/deaggregator_test.rs' panicked at 'ran out of mir dump output to match against.Did not find expected line:" _0 = Baz { x: _3, y: const F32(0), z: const false };"Expected: bb0:{
_2 = _1;
_3 = _2;
_0 = Baz{x: _3,y:constF32(0),z:constfalse};return;}Actual:fnbar(_1:usize) -> Baz{letmut _0:Baz;
scope 1{let _2: usize;}letmut _3:usize;bb0:{StorageLive(_2);
_2 = _1;StorageLive(_3);
_3 = _2;(_0.0: usize) = _3;(_0.1:f32) = constF32(0);(_0.2: bool) = constfalse;StorageDead(_3);StorageDead(_2);return;}}', src/tools/compiletest/src/runtest.rs:2302
note:Run with `RUST_BACKTRACE=1` for a backtrace.

[michaelwoerister]

@bors r+

[bors]

📌 Commit 90c5b71 has been approved by michaelwoerister