add uitests for cargo rust-version field

tests/ui-cargo/cargo_rust_version/warn_both_diff/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "warn-both-diff"
+version = "0.1.0"
+rust-version = "1.56.0"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

tests/ui-cargo/cargo_rust_version/warn_both_diff/clippy.toml

@@ -0,0 +1 @@
+msrv = "1.13"

tests/ui-cargo/cargo_rust_version/warn_both_diff/src/main.rs

@@ -0,0 +1,11 @@
+#![deny(clippy::use_self)]
+
+pub struct Foo;
+
+impl Foo {
+    pub fn bar() -> Foo {
+        Foo
+    }
+}
+
+fn main() {}

tests/ui-cargo/cargo_rust_version/warn_both_diff/src/main.stderr

@@ -0,0 +1,4 @@
+warning: the MSRV in `clippy.toml` and `Cargo.toml` differ; using `1.13.0` from `clippy.toml`
+
+warning: 1 warning emitted
+