Fixed lifetime issue in ast transform tests

[lamarrr]

The ast operation takes references to the expressions. The expression was being copied to a lambda scope and destroyed at the end, leading to a dangling reference.
New code should use the ast tree for constructing complicated expression trees as it handles lifetime management.

Closes #17274

Description

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

New code should use the ast tree for constructing complicated expression trees as it handles lifetime management.

Would it be better to convert this test to use the new approach?

[bdice]

Approving as a minimal fix, but we should evaluate refactoring the tests to use the new machinery that manages expression lifetimes.

[davidwendt]

Verified this fixes the debug build gtest error.

[lamarrr]

New code should use the ast tree for constructing complicated expression trees as it handles lifetime management.

Would it be better to convert this test to use the new approach?

yes, that's the right thing to do. I'll do that in another merge request

[lamarrr]

/merge