Sporadically failing CI testcases

首先运行为 session::tests::test_read_line 提供的 cargo nextest 循环，并检查 src/session.rs:437 处失败的断言。跟踪测试在该断言附近的行为，并通过重复运行确认间歇性结果已被消除，且测试能够在 CI 中可靠通过。

We have some sporadically failing testcases, which blocks CI.
Has been discussed in #101 among other PRs. But I'm opening an issue to keep the discussion in one place.

I'm able to reliably reproduce the failure on my machine like so:

I=1; while cargo nextest run test_read_line; do I=$(($I+1)); echo "PASS $I"; done; echo "FAIL $I"

    Starting 1 tests across 1 binaries (19 skipped)
        FAIL [   0.207s] rexpect session::tests::test_read_line

--- STDOUT:              rexpect session::tests::test_read_line ---

running 1 test
test session::tests::test_read_line ... FAILED

failures:

failures:
    session::tests::test_read_line

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 19 filtered out; finished in 0.21s


--- STDERR:              rexpect session::tests::test_read_line ---
thread 'session::tests::test_read_line' panicked at 'assertion failed: `(left == right)`
  left: `"hans"`,
 right: `"hanshans"`', src/session.rs:437:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

   Canceling due to test failure: 0 tests still running
------------
     Summary [   0.207s] 1 tests run: 0 passed, 1 failed, 19 skipped
        FAIL [   0.207s] rexpect session::tests::test_read_line
error: test run failed
FAIL 270

It failed after 270 runs of the test. I'm running Ubuntu 20.