Add PROC_MACRO_TEST_TOOLCHAIN environment variable

[fasterthanlime]

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

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Sample usage.

Testing the current ABI (variable unset/empty):

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:

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:

❯ 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):

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

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

[Veykril]

@bors r+

[bors]

📌 Commit dcd52ec has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit dcd52ec with merge fa883cb...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing fa883cb to master...