feature: adds fixtures to support multiple files from string

[baszalmstra]

Adds the ability to construct test fixtures that consist of multiple files from a single string. A fixture with multiple files looks like this:

Fixture::parse(
    r#"
    //- /foo.mun
    fn hello_world() {
    }

    //- /bar.mun
    fn baz() {
    }
"#
)

This is used by the MockDatabase to enable creating tests for multiple files.

[Wodann]

Other than some grammar mistakes, this looks great! 👍

[Wodann]

Suggested change

	/// A `Fixtures` describes an single file in a project workspace. `Fixture`s can be parsed from a
	/// A `Fixture` describes a multiple files in a project workspace. `Fixture`s can be parsed from a

[baszalmstra]

No thats actually correct. The struct itself describes a single file in a project workspace. When calling parse a collection is returned.

[Wodann]

The fixtures (plural class name) part or the singular file? It seems like you define multiple Mun files in your fixture, not a single one?

[baszalmstra]

It should be A Fixture describes a single file in a project workspace. Does that make sense?

[Wodann]

I guess my understanding of what a fixture is, is wrong then. I thought that the following snippet contained one fixture, comprised of two files.

Fixture::parse(
    r#"
    //- /foo.mun
    fn hello_world() {
    }

    //- /bar.mun
    fn baz() {
    }
"#
)

[baszalmstra]

Parsing the given strings returns a Vec<Fixture>. One entry for foo.mun and one for bar.mun.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@575a3ba). Click here to learn what that means.
The diff coverage is 89.58%.

@@            Coverage Diff            @@
##             master     #272   +/-   ##
=========================================
  Coverage          ?   79.10%           
=========================================
  Files             ?      219           
  Lines             ?    13024           
  Branches          ?        0           
=========================================
  Hits              ?    10303           
  Misses            ?     2721           
  Partials          ?        0           

Impacted Files	Coverage Δ
crates/mun_hir/src/expr/validator/tests.rs	100.00% <ø> (ø)
crates/mun_hir/src/item_tree/tests.rs	100.00% <ø> (ø)
crates/mun_hir/src/tests.rs	84.61% <ø> (ø)
crates/mun_hir/src/ty/tests.rs	96.38% <ø> (ø)
crates/mun_hir/src/mock.rs	76.92% <75.00%> (ø)
crates/mun_hir/src/fixture.rs	81.81% <81.81%> (ø)
crates/mun_test/src/fixture.rs	92.85% <92.85%> (ø)
crates/mun_memory/src/mapping.rs	96.59% <0.00%> (ø)
crates/mun_diagnostics/src/hir/missing_fields.rs	0.00% <0.00%> (ø)
...tes/mun_language_server/src/protocol/connection.rs	29.16% <0.00%> (ø)
... and 216 more

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[baszalmstra]

Can we merge this @Wodann ?