cargo: stop cargo install from installing fake-editor & co.

Among other things, this prevented `jj` from working with
`cargo binstall`.

The trick is taken from
rust-lang/cargo#2911 (comment).

We could now also remove `--bin jj` from the installation commands
in `install-and-setup.md`, but I'm not sure we should. That argument
makes it clear that the binary is `jj`, not `jj-cli`.

Fixes martinvonz#216.

cli/Cargo.toml

@@ -19,10 +19,12 @@ path = "src/main.rs"
 [[bin]]
 name = "fake-editor"
 path = "testing/fake-editor.rs"
+required-features = ["test_utils"]
 
 [[bin]]
 name = "fake-diff-editor"
 path = "testing/fake-diff-editor.rs"
+required-features = ["test_utils"]
 
 [build-dependencies]
 cargo_metadata = { workspace = true }
@@ -72,10 +74,13 @@ assert_matches = { workspace = true }
 insta = { workspace = true }
 test-case = { workspace = true }
 testutils = { workspace = true }
+# https://github.com/rust-lang/cargo/issues/2911#issuecomment-1483256987
+jj-cli = { path = ".", features = ["test_utils"], default-features = false }
 
 [features]
 default = ["watchman"]
 bench = ["dep:criterion"]
 packaging = []
+test_utils = []
 vendored-openssl = ["git2/vendored-openssl", "jj-lib/vendored-openssl"]
 watchman = ["jj-lib/watchman"]