Visualize scanner related tests (#20412)

This PR improves the scanner related tests a bit. Right now all the
scanner tests (a lot of them) are somewhat hard to reason about and to
figure out whether these are actually correct or not.

With this PR, the existing are extended with a visualization of the file
system tree, but with indicators to know whether or not a file was
scanned or not. If a `.gitignore` is present, it's content is also shown
to make it easier to see why some files are included or not included.

One of the outputs look like this:
```
.
├── ✗ .gitignore
│       src/ignored-by-gitignore.html
└── ✓ src
    ├── ✗ ignore-by-extension.bin
    ├── ✓ ignored-by-gitignore.html
    ├── ✗ ignored-by-source-not.html
    ├── ✓ keep-me.html
    └── ✓ new-file.html
```

I'm sure we can cleanup a lot of these tests, simplify, deduplicate, but
I didn't want to do that as part of this PR. This PR is purely adding
this visualization.

This uses the [insta.rs](https://insta.rs) library for (inline) snapshot
tests. If at some point the snapshots change, then you get a nice UI:
<img width="958" height="670" alt="image"
src="https://github.com/user-attachments/assets/652cf48f-e1b1-4fe5-a42a-29c5a888063a"
/>


## Test plan

1. All tests should pass [ci-all]
2. Nothing in the code itself changed, so nothing should break

---

<sub>Stack created with <a
href="https://github.com/github/gh-stack">GitHub Stacks CLI</a> • <a
href="https://gh.io/stacks-feedback">Give Feedback 💬</a></sub>

Cargo.lock

@@ -59,6 +59,17 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "console"
+version = "0.16.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4fe5f465a4f6fee88fad41b85d990f84c835335e85b5d9e6e63e0d06d28cba7c"
+dependencies = [
+ "encode_unicode",
+ "libc",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "convert_case"
 version = "0.11.0"
@@ -126,6 +137,12 @@ version = "1.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7fcaabb2fef8c910e7f4c7ce9f67a1283a1715879a7c230ca9d6d1ae31f16d91"
 
+[[package]]
+name = "encode_unicode"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "34aa73646ffb006b8f5147f3dc182bd4bcb190227ce861fc4a4844bf8e3cb2c0"
+
 [[package]]
 name = "errno"
 version = "0.3.9"
@@ -296,6 +313,18 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "insta"
+version = "1.48.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "86f0f8fee8c926415c58d6ae43a08523a26faccb2323f5e6b644fe7dd4ef6b82"
+dependencies = [
+ "console",
+ "once_cell",
+ "similar",
+ "tempfile",
+]
+
 [[package]]
 name = "itertools"
 version = "0.11.0"
@@ -445,9 +474,9 @@ dependencies = [
 
 [[package]]
 name = "once_cell"
-version = "1.19.0"
+version = "1.21.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3fdb12b2476b595f9358c5161aa467c2438859caa136dec86c26fdd2efe17b92"
+checksum = "9f7c3e4beb33f85d45ae3e3a1792185706c8e16d043238c593331cc7cd313b50"
 
 [[package]]
 name = "overload"
@@ -602,6 +631,12 @@ dependencies = [
  "lazy_static",
 ]
 
+[[package]]
+name = "similar"
+version = "2.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bbbb5d9659141646ae647b42fe094daf6c6192d1620870b449d9557f748b2daa"
+
 [[package]]
 name = "slab"
 version = "0.4.12"
@@ -647,6 +682,7 @@ dependencies = [
  "fast-glob",
  "globwalk",
  "ignore 0.4.33",
+ "insta",
  "log",
  "pretty_assertions",
  "rayon",
@@ -832,6 +868,15 @@ dependencies = [
  "windows-targets",
 ]
 
+[[package]]
+name = "windows-sys"
+version = "0.61.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ae137229bcbd6cdf0f7b80a31df61766145077ddf49416a728b02cb3921ff3fc"
+dependencies = [
+ "windows-link",
+]
+
 [[package]]
 name = "windows-targets"
 version = "0.52.6"

crates/oxide/Cargo.toml

@@ -20,6 +20,7 @@ ignore = { path = "../ignore" }
 regex = "1.11.1"
 
 [dev-dependencies]
+insta = "1.48.0"
 tempfile = "3.13.0"
 pretty_assertions = "1.4.1"
 unicode-width = "0.2.0"