Add min_const_gen to "all stable features" test invocation

`min_const_gen` is a stable feature, but was not added to this test
invocation when it was introduced.

Unblocks #1173.

Author: connec

.github/workflows/test.yml

@@ -83,7 +83,7 @@ jobs:
           cargo build --target ${{ matrix.target }} --no-default-features --features alloc,getrandom,small_rng
           cargo test --target ${{ matrix.target }} --lib --tests --no-default-features --features=alloc,getrandom,small_rng
           # all stable features:
-          cargo test --target ${{ matrix.target }} --features=serde1,log,small_rng
+          cargo test --target ${{ matrix.target }} --features=serde1,log,small_rng,min_const_gen
           cargo test --target ${{ matrix.target }} --examples
       - name: Test rand_core
         run: |