Avoid mysterious Default::default()

In `deploy::test::{low_level_simple, low_level_skip}`,
`Default::default()` was used to construct a `BTreeMap`, with the only
indication of the variable's type being the signature of
`RealActionRunner::new()`, which it was passed by reference to.

Clippy's default-trait-access lint caught this.

src/deploy.rs

@@ -697,7 +697,7 @@ mod test {
 
         let opt = Options::default();
         let handlebars = handlebars::Handlebars::new();
-        let variables = Default::default();
+        let variables = BTreeMap::new();
 
         // Expectation:
         // create_symlink
@@ -800,7 +800,7 @@ mod test {
 
         let opt = Options::default();
         let handlebars = handlebars::Handlebars::new();
-        let variables = Default::default();
+        let variables = BTreeMap::new();
 
         // Expectation:
         // create_symlink