avoid recompiling miri for 2nd test run; avoid unreadable output due to backtraces

Author: RalfJung

.travis.yml

@@ -26,7 +26,7 @@ script:
 - |
   # Test and install plain miri
   cargo build --release --all-features &&
-  RUST_BACKTRACE=1 cargo test --release --all-features --all &&
+  cargo test --release --all-features &&
   cargo install --all-features --force
 - |
   # test that the rustc_tests binary compiles
@@ -55,7 +55,7 @@ script:
   cd ..
 - |
   # and run all tests with full mir
-  cargo test --release
+  cargo test --release --all-features
 notifications:
   email:
     on_success: never