Use workspace.default-members to specify default crates for 'cargo build'

… and 'cargo test', etc. Include Servo and its unit tests,
but not Stylo because that would try to compile the style
crate with incompatible feature flags:
https://github.com/rust-lang/cargo/issues/4463

`workspace.default-members` was added in
https://github.com/rust-lang/cargo/pull/4743.
Older Cargo versions ignore it.

Cargo.toml

@@ -8,6 +8,10 @@ members = [
 
     "ports/cef",
 ]
+default-members = [
+    "ports/servo",
+    "tests/unit/*",
+]
 exclude = [".cargo"]
 
 [profile.dev]