Fix unexpected expansion

[stephencelis]

This contains swift-snapshot-testing updates that should hopefully fix #6.

@gohanlon Can you give this branch a try and let us know if it fixes things for you?

[gohanlon]

Thanks @stephencelis! This does fix #6 for me: No crash, and now I get nice test failures. 🎉

---

Here's what I did to verify the fix:

After adopting this PR, starting with this (malformed) test:

  func testVarPropertyWithInitializerWithoutExplicitType_FailsWithDiagnostic() {
    assertMacro {
      """
      @MemberwiseInit
      public struct Pedometer {
        var stepsToday = 0
      }
      """
    } expansion: {
      """
      @MemberwiseInit
      public struct Pedometer {
        var stepsToday = 0
            ┬─────────────
            ╰─ 🛑 @MemberwiseInit requires explicit type declarations for `var` stored properties.
      }
      """
    }
  }

I then run the test, which re-records automatically as follows:

  func testVarPropertyWithInitializerWithoutExplicitType_FailsWithDiagnostic() {
    assertMacro {
      """
      @MemberwiseInit
      public struct Pedometer {
        var stepsToday = 0
      }
      """
    } diagnostics: {
      """
      @MemberwiseInit
      public struct Pedometer {
        var stepsToday = 0
            ┬─────────────
            ╰─ 🛑 @MemberwiseInit requires explicit type declarations for `var` stored properties.
      }
      """
    } expansion: {
      """
      @MemberwiseInit
      public struct Pedometer {
        var stepsToday = 0
            ┬─────────────
            ╰─ 🛑 @MemberwiseInit requires explicit type declarations for `var` stored properties.
      }
      """
    }
  }

As I'd expect, the recording run has the following two test failures:

…/gohanlon/swift-memberwise-init-macro/Tests/MemberwiseInitTests/MemberwiseInitTests.swift:276 testVarPropertyWithInitializerWithoutExplicitType_FailsWithDiagnostic():
🛑 failed - Automatically recorded a new snapshot for "diagnostics".

Re-run "testVarPropertyWithInitializerWithoutExplicitType_FailsWithDiagnostic()" to assert against the newly-recorded snapshot.

and:

…/gohanlon/swift-memberwise-init-macro/Tests/MemberwiseInitTests/MemberwiseInitTests.swift:276 testVarPropertyWithInitializerWithoutExplicitType_FailsWithDiagnostic():
🛑 failed - Expected macro expansion, but there was none

Then I ran the test again and, as I'd expect, I get just the second failure.