Raise minimum compiler for test suite to rust 1.70

With 1.61:

    error: package `indexmap v2.6.0` cannot be built because it requires
    rustc 1.63 or newer, while the currently active rustc version is 1.61.0

With 1.63:

    error: package `toml_datetime v0.6.8` cannot be built because it requires
    rustc 1.65 or newer, while the currently active rustc version is 1.63.0

With 1.65:

    error: package `trybuild v1.0.101` cannot be built because it requires
    rustc 1.70 or newer, while the currently active rustc version is 1.65.0
    Either upgrade to rustc 1.70 or newer, or use
    cargo update -p [email protected] --precise ver
    where `ver` is the latest version of `trybuild` supporting rustc 1.65.0

Author: dtolnay

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.61.0]
+        rust: [nightly, beta, stable, 1.70.0, 1.61.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v4
@@ -39,6 +39,7 @@ jobs:
         run: echo RUSTFLAGS=${RUSTFLAGS}\ --cfg=thiserror_nightly_testing >> $GITHUB_ENV
         if: matrix.rust == 'nightly'
       - run: cargo test --all
+        if: matrix.rust != '1.61.0'
       - uses: actions/upload-artifact@v4
         if: matrix.rust == 'nightly' && always()
         with: