Auto merge of #148424 - Zalathar:tests, r=Kobzol

bootstrap: Add snapshot tests for path-to-step handling

This PR adds a suite of snapshot tests for how bootstrap translates command-line “paths” (which are not necessarily actual paths) into a set of top-level steps to execute.

Unlike the existing suite of snapshot tests (for transitive step logging), I decided to use `.snap` files over inline snapshots, because I find that inline snapshots quickly make test files impossible to navigate. Instead, I set up a macro that makes it relatively easy to add or modify test cases.

Using `.snap` files also means that the tests can be blessed by setting `INSTA_UPDATE=always`, without necessarily needing the `cargo insta` tool installed, though the tool can still be used instead if desired.

These snapshot tests capture _current_ behavior, to prevent unintended changes or regressions. If the current behavior is wrong or undersirable, then any fix will necessarily have to re-bless the affected tests!

r? Kobzol

Author: bors