Make tests configuration option optional.

Bug: 293289578
Test: atest cargo_embargo.test
Change-Id: Ia35acdcad920e0f585cc5be650f1d1e3b6aa176d

tools/cargo_embargo/src/config.rs

@@ -33,6 +33,7 @@ fn default_true() -> bool {
 #[serde(deny_unknown_fields)]
 pub struct Config {
     /// Whether to output "rust_test" modules.
+    #[serde(default)]
     pub tests: bool,
     /// Set of features to enable. If non-empty, disables the default crate features.
     #[serde(default)]

tools/cargo_embargo/testdata/plotters/cargo_embargo.json

@@ -1,6 +1,5 @@
 {
   "features": ["svg_backend", "area_series", "line_series"],
-  "tests": false,
   "package": {
     "plotters": {
       "device_supported": true