red_knot_test: add support for diagnostic snapshotting

This ties together everything from the previous commits.
Some interesting bits here are how the snapshot is generated
(where we include relevant info to make it easier to review
the snapshots) and also a tweak to how inline assertions are
processed.

This commit also includes some example snapshots just to get
a sense of what they look like. Follow-up work should add
more of these I think.

crates/red_knot_python_semantic/resources/mdtest/import/basic.md

@@ -116,8 +116,18 @@ reveal_type(c.C)  # revealed: Literal[C]
 class C: ...
 ```
 
+## Unresolvable module import
+
+<!-- snapshot-diagnostics -->
+
+```py
+import zqzqzqzqzqzqzq  # error: [unresolved-import] "Cannot resolve import `zqzqzqzqzqzqzq`"
+```
+
 ## Unresolvable submodule imports
 
+<!-- snapshot-diagnostics -->
+
 ```py
 # Topmost component resolvable, submodule not resolvable:
 import a.foo  # error: [unresolved-import] "Cannot resolve import `a.foo`"

crates/red_knot_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Unresolvable_module_import.snap

@@ -0,0 +1,28 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Unresolvable module import
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet__1.py
+
+```
+1 | import zqzqzqzqzqzqzq  # error: [unresolved-import] "Cannot resolve import `zqzqzqzqzqzqzq`"
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet__1.py:1:8
+  |
+1 | import zqzqzqzqzqzqzq  # error: [unresolved-import] "Cannot resolve import `zqzqzqzqzqzqzq`"
+  |        ^^^^^^^^^^^^^^ Cannot resolve import `zqzqzqzqzqzqzq`
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Unresolvable_submodule_imports.snap

@@ -0,0 +1,51 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Unresolvable submodule imports
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet__1.py
+
+```
+1 | # Topmost component resolvable, submodule not resolvable:
+2 | import a.foo  # error: [unresolved-import] "Cannot resolve import `a.foo`"
+3 | 
+4 | # Topmost component unresolvable:
+5 | import b.foo  # error: [unresolved-import] "Cannot resolve import `b.foo`"
+```
+
+## a/__init__.py
+
+```
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet__1.py:2:8
+  |
+1 | # Topmost component resolvable, submodule not resolvable:
+2 | import a.foo  # error: [unresolved-import] "Cannot resolve import `a.foo`"
+  |        ^^^^^ Cannot resolve import `a.foo`
+3 |
+4 | # Topmost component unresolvable:
+  |
+
+```
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet__1.py:5:8
+  |
+4 | # Topmost component unresolvable:
+5 | import b.foo  # error: [unresolved-import] "Cannot resolve import `b.foo`"
+  |        ^^^^^ Cannot resolve import `b.foo`
+  |
+
+```