fix oversight in my cli test system: it needed to lookup target binary, not our PATH.

(This functionality is also available via other means, such as
`$CARGO_BIN_EXE_<name>` and https://crates.io/crates/assert_cmd. I opted not to
use the builtin env variable because that is only available in very recent cargo
versions, and I would prefer our test suite to work peven on older versions of
cargo, if that is feasible...)

Author: pnkfelix

Cargo.toml

@@ -36,3 +36,4 @@ colored="1.9"
 [dev-dependencies]
 quickcheck = "0.9.2"
 tempfile = "3.1.0"
+test_bin=  "0.3.0"

tests/common/command_invocation.rs

@@ -1,5 +1,5 @@
 use std::path::Path;
-use std::process::{Command, ExitStatus};
+use std::process::ExitStatus;
 
 pub(crate) struct Context<'a> {
     pub cli_params: &'a [&'a str],
@@ -14,14 +14,16 @@ pub struct Output {
 
 impl<'a> Context<'a> {
     pub fn run(&mut self) -> Result<Output, failure::Error> {
-        let mut command = Command::new("cargo-bisect-rustc");
+        let mut command = test_bin::get_test_bin("cargo-bisect-rustc");
         for param in self.cli_params {
             command.arg(param);
         }
         let dir = self.dir;
-        println!("running `cargo-bisect-rustc {}` in {:?}",
+        println!("running `{:?} {}` in {:?}",
+                 command,
                  self.cli_params.join(" "),
                  dir.display());
+        assert!(dir.exists());
         let output = command.current_dir(dir).output()?;
 
         let stderr = String::from_utf8_lossy(&output.stderr);