Fixes of unified rc5

[forsyth2]

Summary

Objectives:

• Note: the first two commits are from Isolate hsi calls #440, and will be merged separately [EDIT: now rebased away]
• Commit 3: Bump to v1.6.0rc2
• Commits 4-5: Fixes to hsi calls in tests
• Commit 6: Adding Improv support

Issue resolution:

• Resolve issues encountered while testing zstash v1.6.0 RCs (E3SM Unified 1.13.0 RCs). See here.

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
  • I'm actually going to use a merge commit since the commits represent distinct fixes/updates.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.

[forsyth2]

Same as #440, this was tested more completely as part of the E3SM Unified 1.13.0rc5 testing, which included zstash v1.6.0rc2, so we can merge now.