test: avoid hardcoded target spec json

Read a real target spec JSON so we no longer need to hardcode
a target spec JSON here.
Cargo itself should not care about the spec schema.

Let's stop bothering compiler contributors.

Author: weihanglo

tests/testsuite/rustc.rs

@@ -818,17 +818,23 @@ windows
 
 #[cargo_test]
 fn rustc_with_print_cfg_config_toml_env() {
+    // We read from a built-in target json spec, so schema change won't block
+    // compiler contributors on fixing failing Cargo tests.
+    let target_json_spec = std::process::Command::new("rustc")
+        .env("RUSTC_BOOTSTRAP", "1")
+        .arg("--print")
+        .arg("target-spec-json")
+        .arg("-Zunstable-options")
+        .arg("--target")
+        .arg("x86_64-unknown-none")
+        .output()
+        .unwrap()
+        .stdout;
+    let target_json_spec = String::from_utf8_lossy(&target_json_spec);
+
     let p = project()
         .file("Cargo.toml", &basic_bin_manifest("foo"))
-        .file(
-            "targets/best-target.json",
-            r#"{
-  "llvm-target": "x86_64-unknown-none",
-  "target-pointer-width": "64",
-  "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
-  "arch": "x86_64"
-}"#,
-        )
+        .file("targets/best-target.json", &target_json_spec)
         .file(
             ".cargo/config.toml",
             r#"