Auto merge of #12834 - fasterthanlime:proc-macro-test-toolchain, r=Veykril

Auto merge of #12834 - fasterthanlime:proc-macro-test-toolchain, r=Veykril

Add PROC_MACRO_TEST_TOOLCHAIN environment variable

This allows overriding the toolchain used to run `proc-macro-srv` tests.

---

Sample usage.

Testing the current ABI (variable unset/empty):

```shell
amos@tails ~/bearcove/rust-analyzer/crates/proc-macro-srv proc-macro-test-toolchain*
❯ PROC_MACRO_TEST_TOOLCHAIN="" cargo test --quiet

running 16 tests
................
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s
```

Testing an older ABI:

```shell
amos@tails ~/bearcove/rust-analyzer/crates/proc-macro-srv proc-macro-test-toolchain*
❯ PROC_MACRO_TEST_TOOLCHAIN="1.58" cargo test --quiet

running 16 tests
................
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s
```

Testing current nightly ABI:

```shell
❯ rustc +nightly --version
rustc 1.64.0-nightly (f8588549c 2022-07-18)

❯ PROC_MACRO_TEST_TOOLCHAIN="nightly" cargo test --quiet

running 16 tests
................
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s
```

Testing future ABI (`rust-lang/rust` master):

```shell
amos@tails ~/bearcove/rust-analyzer/crates/proc-macro-srv proc-macro-test-toolchain
❯ PROC_MACRO_TEST_TOOLCHAIN="stage1" cargo test --quiet

running 16 tests
..........thread '<unnamed>' panicked at 'range end index 216221164920373249 out of range for slice of length 18', library/core/src/slice/index.rs:73:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
....F.
failures:

---- tests::test_fn_like_macro2 stdout ----
thread 'tests::test_fn_like_macro2' panicked at 'called `Result::unwrap()` on an `Err` value: "range end index 216221164920373249 out of range for slice of length 18"', crates/proc-macro-srv/src/tests/utils.rs:38:83
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
    tests::test_fn_like_macro2

test result: FAILED. 15 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass '--lib
```

---

Tagging `@jonas-schievink:` this might be helpful when updating versioned ABIs later on.