Change env var used for testing Clippy

This changes the variable used for testing Clippy in the internal test
suite:

```
CLIPPY_TESTS -> __CLIPPY_INTERNAL_TESTS
```

`CLIPPY_TESTS` is understandably used in environments of Clippy users,
so we shouldn't use it in our test suite.

Author: flip1995

doc/adding_lints.md

@@ -147,10 +147,14 @@ add `// edition:2018` at the top of the test file (note that it's space-sensitiv
 
 Manually testing against an example file can be useful if you have added some
 `println!`s and the test suite output becomes unreadable. To try Clippy with
-your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
-clippy-driver -- -L ./target/debug input.rs` from the working copy root.
+your local modifications, run
 
-With tests in place, let's have a look at implementing our lint now.
+```
+env __CLIPPY_INTERNAL_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs
+```
+
+from the working copy root. With tests in place, let's have a look at
+implementing our lint now.
 
 ## Lint declaration
 

src/driver.rs

@@ -298,7 +298,7 @@ pub fn main() {
         // - IF Clippy is run on the main crate, not on deps (`!cap_lints_allow`) THEN
         //    - IF `--no-deps` is not set (`!no_deps`) OR
         //    - IF `--no-deps` is set and Clippy is run on the specified primary package
-        let clippy_tests_set = env::var("CLIPPY_TESTS").map_or(false, |val| val == "true");
+        let clippy_tests_set = env::var("__CLIPPY_INTERNAL_TESTS").map_or(false, |val| val == "true");
         let cap_lints_allow = arg_value(&orig_args, "--cap-lints", |val| val == "allow").is_some();
         let in_primary_package = env::var("CARGO_PRIMARY_PACKAGE").is_ok();
 

tests/compile-test.rs

@@ -254,7 +254,7 @@ fn run_ui_cargo(config: &mut compiletest::Config) {
 
 fn prepare_env() {
     set_var("CLIPPY_DISABLE_DOCS_LINKS", "true");
-    set_var("CLIPPY_TESTS", "true");
+    set_var("__CLIPPY_INTERNAL_TESTS", "true");
     //set_var("RUST_BACKTRACE", "0");
 }