Change loom tests to use cfg(loom) internally (#314)

ci/azure-loom.yml

@@ -0,0 +1,15 @@
+jobs:
+- job: ${{parameters.name}}
+  displayName: Loom tests
+  pool:
+    vmImage: ubuntu-16.04
+
+  steps:
+  - template: azure-install-rust.yml
+    parameters:
+      rust_version: ${{parameters.rust_version}}
+
+  - script: RUSTFLAGS="--cfg loom" cargo test --lib
+    displayName: RUSTFLAGS="--cfg loom" cargo test --lib
+
+