test helpers: do not use default build system

Many of the executeSwift* command helpers accept a build system
provider, which was defaulted to the native build system.  Since we are
augmenting the tests to also run against the SwiftBuild, setting a
default value may lead to an undesirable test behaviour.

Update all the executeSwift* commands to no set a default build system
provider.

Author: bkhouri

Fixtures/Miscellaneous/Plugins/CommandPluginUnsuccessful/Package.swift

@@ -0,0 +1,30 @@
+// swift-tools-version: 5.6
+import PackageDescription
+let package = Package(
+    name: "MyPackage",
+    products: [
+        .library(
+            name: "MyLibrary",
+            targets: ["MyLibrary"]
+        ),
+        .executable(
+            name: "MyExecutable",
+            targets: ["MyExecutable"]
+        ),
+    ],
+    targets: [
+        .target(
+            name: "MyLibrary"
+        ),
+        .executableTarget(
+            name: "MyExecutable",
+            dependencies: ["MyLibrary"]
+        ),
+        .plugin(
+            name: "MyCommandPlugin",
+            capability: .command(
+                intent: .custom(verb: "my-build-tester", description: "Help description")
+            )
+        ),
+    ]
+)

Fixtures/Miscellaneous/Plugins/CommandPluginUnsuccessful/Plugins/MyCommandPlugin/plugin.swift

@@ -0,0 +1,9 @@
+import PackagePlugin
+@main struct MyCommandPlugin: CommandPlugin {
+    func performCommand(
+        context: PluginContext,
+        arguments: [String]
+    ) throws {
+        this is an error
+    }
+}

Fixtures/Miscellaneous/Plugins/CommandPluginUnsuccessful/Sources/MyExecutable/main.swift

@@ -0,0 +1,2 @@
+import MyLibrary
+print("\\(GetGreeting()), World!")

Fixtures/Miscellaneous/Plugins/CommandPluginUnsuccessful/Sources/MyLibrary/library.swift

@@ -0,0 +1 @@
+public func GetGreeting() -> String { return "Hello" }

Fixtures/Miscellaneous/Plugins/PluginSuccessful/Package.swift

@@ -0,0 +1,34 @@
+// swift-tools-version: 5.6
+import PackageDescription
+let package = Package(
+    name: "MyPackage",
+    products: [
+        .library(
+            name: "MyLibrary",
+            targets: ["MyLibrary"]
+        ),
+        .executable(
+            name: "MyExecutable",
+            targets: ["MyExecutable"]
+        ),
+    ],
+    targets: [
+        .target(
+            name: "MyLibrary"
+        ),
+        .executableTarget(
+            name: "MyExecutable",
+            dependencies: ["MyLibrary"]
+        ),
+        .plugin(
+            name: "MyBuildToolPlugin",
+            capability: .buildTool()
+        ),
+        .plugin(
+            name: "MyCommandPlugin",
+            capability: .command(
+                intent: .custom(verb: "my-build-tester", description: "Help description")
+            )
+        ),
+    ]
+)

Fixtures/Miscellaneous/Plugins/PluginSuccessful/Plugins/MyBuildToolPlugin/plugin.swift

@@ -0,0 +1,9 @@
+import PackagePlugin
+@main struct MyBuildToolPlugin: BuildToolPlugin {
+    func createBuildCommands(
+        context: PluginContext,
+        target: Target
+    ) throws -> [Command] {
+        return []
+    }
+}

Fixtures/Miscellaneous/Plugins/PluginSuccessful/Plugins/MyCommandPlugin/plugin.swift

@@ -0,0 +1,8 @@
+import PackagePlugin
+@main struct MyCommandPlugin: CommandPlugin {
+    func performCommand(
+        context: PluginContext,
+        arguments: [String]
+    ) throws {
+    }
+}

Fixtures/Miscellaneous/Plugins/PluginSuccessful/Sources/MyExecutable/main.swift

@@ -0,0 +1,2 @@
+import MyLibrary
+print("\\(GetGreeting()), World!")

Fixtures/Miscellaneous/Plugins/PluginSuccessful/Sources/MyLibrary/library.swift

@@ -0,0 +1 @@
+public func GetGreeting() -> String { return "Hello" }

Fixtures/Miscellaneous/Unicode/Package.swift

@@ -13,7 +13,7 @@ let complicatedString = "πשּׁµ𝄞🇺🇳🇮🇱x̱̱̱̱̱̄̄̄̄̄"
 // 🇮🇱 U+1F1EE U+1F1F1: Second consecutive regional indicator. (Complicated grapheme breaking.)
 // x̱̱̱̱̱̄̄̄̄̄ U+0078 (U+0331 U+0304) × 5: Extremely long combining sequence. (Also reordrant under normalization.)
 
-// The following verifies that sources haven’t been normalized, which would reduce the test’s effectiveness.
+// The following verifies that sources haven't been normalized, which would reduce the test's effectiveness.
 var verify = "\u{03C0}\u{0FB2C}\u{00B5}\u{1D11E}\u{1F1FA}\u{1F1F3}\u{1F1EE}\u{1F1F1}\u{0078}\u{0331}\u{0304}\u{0331}\u{0304}\u{0331}\u{0304}\u{0331}\u{0304}\u{0331}\u{0304}"
 assert(
     complicatedString.unicodeScalars.elementsEqual(verify.unicodeScalars),