fix(tests): correct inverted assertions for read-only snapshot property editing

[mvanhorn]

Checklist

• I have read the contributing guidelines and attest that all commits in this PR are signed off, cryptographically signed, and follow this project's commit structure.
• I have checked and added or updated relevant documentation.

Summary

• test/suites/snapshots.sh and test/suites/storage_snapshots.sh both contain inverted test assertions that cause the tests to pass when editing read-only snapshot properties silently succeeds (the broken behavior) and fail when the API correctly rejects such edits.
• The pipeline ! cmd 2>&1 | grep -xF "$MSG" || false negates grep's exit code, so the test passes when the error message is absent — exactly the opposite of the intended assertion.
• Replaces the inverted pipeline with the pattern identified by @simondeziel in issue Editing read-only properties is silently ignored #18398: capture output into a variable, assert the command exits non-zero, then assert the expected error string is present.

Why this matters

The inverted assertions mean the regression introduced in #17070 (addressing #14423) was never caught by the test suite. A member maintainer (@simondeziel) diagnosed the flaw and provided the corrected pattern in the issue thread. This PR applies that pattern to both affected test files.

Testing

These are shell test files with no production code changes. Bash syntax has been verified (bash -n). The corrected assertions will:

• Fail when lxc config edit foo/snap0 with a modified read-only field silently returns exit 0 (regression guard active)
• Pass when the API correctly returns a non-zero exit code with the expected "Only 'expires_at' field(s) can be modified" error message
• Behave identically in the storage_snapshots.sh parallel case for storage volume snapshots
• Leave the positive expires_at-editable test immediately following the loop unaffected

Fixes #18398

[simondeziel]

@mvanhorn thanks for showing interest on working on this! I see that you've provided test fixes but AFAIK, the main issue lies in the Go code.

[simondeziel]

I've put it to draft until the Go code change lands.