Add tests for --json unused-externs

There were none at all. These test for original functionality,
but this also adds a test that `-Dunused-crate-dependencies`
causes a compilation failure, which currently fails
(https://github.com/rust-lang/rust/issues/96068). This is fixed in
subsequent changes.

src/test/ui/unused-crate-deps/deny-attr.rs

@@ -0,0 +1,9 @@
+// Check for unused crate dep, no path
+
+// edition:2018
+// aux-crate:bar=bar.rs
+
+#![deny(unused_crate_dependencies)]
+//~^ ERROR external crate `bar` unused in
+
+fn main() {}

src/test/ui/unused-crate-deps/deny-attr.stderr

@@ -0,0 +1,14 @@
+error: external crate `bar` unused in `deny_attr`: remove the dependency or add `use bar as _;`
+  --> $DIR/deny-attr.rs:6:1
+   |
+LL | #![deny(unused_crate_dependencies)]
+   | ^
+   |
+note: the lint level is defined here
+  --> $DIR/deny-attr.rs:6:9
+   |
+LL | #![deny(unused_crate_dependencies)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/unused-crate-deps/deny-cmdline-json.rs

@@ -0,0 +1,7 @@
+// Check for unused crate dep, no path
+
+// edition:2018
+// compile-flags: -Dunused-crate-dependencies  -Zunstable-options --json unused-externs --error-format=json
+// aux-crate:bar=bar.rs
+
+fn main() {}

src/test/ui/unused-crate-deps/deny-cmdline-json.stderr

@@ -0,0 +1 @@
+{"lint_level":"deny","unused_extern_names":["bar"]}

src/test/ui/unused-crate-deps/deny-cmdline.rs

@@ -0,0 +1,8 @@
+// Check for unused crate dep, no path
+
+// edition:2018
+// compile-flags: -Dunused-crate-dependencies
+// aux-crate:bar=bar.rs
+
+fn main() {}
+//~^ ERROR external crate `bar` unused in

src/test/ui/unused-crate-deps/deny-cmdline.stderr

@@ -0,0 +1,10 @@
+error: external crate `bar` unused in `deny_cmdline`: remove the dependency or add `use bar as _;`
+  --> $DIR/deny-cmdline.rs:7:1
+   |
+LL | fn main() {}
+   | ^
+   |
+   = note: requested on the command line with `-D unused-crate-dependencies`
+
+error: aborting due to previous error
+

src/test/ui/unused-crate-deps/warn-cmdline-json.rs

@@ -0,0 +1,8 @@
+// Check for unused crate dep, no path
+
+// edition:2018
+// check-pass
+// compile-flags: -Wunused-crate-dependencies -Zunstable-options --json unused-externs --error-format=json
+// aux-crate:bar=bar.rs
+
+fn main() {}

src/test/ui/unused-crate-deps/warn-cmdline-json.stderr

@@ -0,0 +1 @@
+{"lint_level":"warn","unused_extern_names":["bar"]}