Set up a single job for completion testing

[shannmu]

I found a issue when running the clap_complete tests locally (on the clap master branch) using the command:

cargo test --features="unstable-dynamic"

The result was:

---- expected: tests/testsuite/fish.rs:186:20
++++ actual:   In-memory
   1      - % exhaustive
        1 + % exhaustive
   2    2 | action                                                             last              -V         (Print version)
   3    3 | alias                                                              pacman            --generate      (generate)
   4    4 | complete            (Register shell completions for this program)  quote             --global      (everywhere)
   5    5 | help  (Print this message or the help of the given subcommand(s))  value             --help        (Print help)
   6    6 | hint                                                               -h  (Print help)  --version  (Print version)∅

Update with SNAPSHOTS=overwrite

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    fish::complete_dynamic

test result: FAILED. 72 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 2.87s

There is a difference of a space between them. However, this test case error was not reported in the CI. The issue is may due to this line

clap/clap_complete/tests/testsuite/fish.rs

Line 146 in 70e8417

if !common::has_command("fish") {

. So I think that we should make sure the job's environment includes bash, fish, elvish, and zsh shells to cover all test cases in CI.