tests: fix system tests on non-nightly

src/config/mod.rs

diff --git a/src/config/mod.rs b/src/config/mod.rs
index 1d0217f..94f390c 100644
--- a/src/config/mod.rs
+++ b/src/config/mod.rs
@@ -476,6 +476,9 @@
 
     #[test]
     fn test_valid_license_template_path() {
+        if !crate::is_nightly_channel!() {
+            return;
+        }
         let toml = r#"license_template_path = "tests/license-template/lt.txt""#;
         let config = Config::from_toml(toml, Path::new("")).unwrap();
         assert!(config.license_template.is_some());

tests/config/issue-3779.toml

diff --git a/tests/config/issue-3779.toml b/tests/config/issue-3779.toml
index 83505f4..6ca52ae 100644
--- a/tests/config/issue-3779.toml
+++ b/tests/config/issue-3779.toml
@@ -1,4 +1,3 @@
-unstable_features = true
 ignore = [
   "tests/**/issue-3779/ice.rs"
 ]

tests/source/issue-3779/lib.rs

diff --git a/tests/source/issue-3779/lib.rs b/tests/source/issue-3779/lib.rs
index d7c2ae2..16e9d48 100644
--- a/tests/source/issue-3779/lib.rs
+++ b/tests/source/issue-3779/lib.rs
@@ -1,3 +1,4 @@
+// rustfmt-unstable: true
 // rustfmt-config: issue-3779.toml
 
 #[path = "ice.rs"]

tests/target/issue-3779/lib.rs

diff --git a/tests/target/issue-3779/lib.rs b/tests/target/issue-3779/lib.rs
index ebebce4..a5673a4 100644
--- a/tests/target/issue-3779/lib.rs
+++ b/tests/target/issue-3779/lib.rs
@@ -1,3 +1,4 @@
+// rustfmt-unstable: true
 // rustfmt-config: issue-3779.toml
 
 #[path = "ice.rs"]