Tidy up trap-handling code.

appveyor.yml

@@ -1,19 +1,15 @@
+os: Visual Studio 2017
 environment:
   matrix:
-  - TARGET: x86_64-pc-windows-gnu
-    BITS: 64
-    MSYS2: 1
   - TARGET: x86_64-pc-windows-msvc
-    BITS: 64
 install:
   - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init.exe -yv --default-host %target%
   - set PATH=%PATH%;%USERPROFILE%\.cargo\bin
-  - if defined MSYS2 set PATH=C:\msys64\mingw%BITS%\bin;%PATH%
   - rustc -vV
   - cargo -vV
   - git submodule update --init --recursive
 build: false
 test_script:
-  - cargo build --verbose --all
+  - set RUST_BACKTRACE=1
   - cargo test --verbose --all