test: Simplify get_exec_path using CARGO_BIN_EXE_toml

This seems to be the way Cargo docs prescribe:
  https://doc.rust-lang.org/cargo/reference/cargo-targets.html#integration-tests

The code comes out much nicer... and it also behaves more robustly,
because Cargo doesn't set CARGO_TARGET_DIR; it reads it, and there
are several other ways to tell Cargo the same thing:
  https://doc.rust-lang.org/cargo/reference/config.html#buildtarget-dir

So this new version will work correctly with, e.g.,
`cargo test --target-dir=…`, while the old version would break.

test/test.rs

@@ -1,5 +1,4 @@
 use std::env;
-use std::ffi::OsString;
 use std::fs;
 use std::path::PathBuf;
 use std::process;
@@ -156,7 +155,7 @@ struct TestCaseState {
 
 impl TestCaseState {
     pub fn new() -> Self {
-        let cmd = process::Command::new(get_exec_path());
+        let cmd = process::Command::new(env!("CARGO_BIN_EXE_toml"));
         let dir = tempfile::tempdir().expect("failed to create tempdir");
         let filename = dir.path().join("test.toml");
         TestCaseState { cmd, dir, filename }
@@ -209,15 +208,6 @@ impl TestCaseState {
     }
 }
 
-fn get_exec_path() -> PathBuf {
-    // TODO is there no cleaner way to get this from Cargo?
-    // Also should it really be "debug"?
-    let target_dir: PathBuf = env::var_os("CARGO_TARGET_DIR")
-        .unwrap_or_else(|| OsString::from("target"))
-        .into();
-    target_dir.join("debug").join("toml")
-}
-
 /// Like `assert!(actual.contains(pattern))`, but with more informative output.
 #[rustfmt::skip]
 fn check_contains(pattern: &str, actual: &str) {