Conditionalize the keyword in the generated test main (#5721)

# Motivation
Right now the code for the test main file contains this generated code

```swift
var tests = [XCTestCaseEntry]()
```

For the rare cases where a package has no tests the generated code produces a warning since the `var tests` was never mutated. If these users are also running `warnings-as-errors` their compile will fail.

# Modification
This PR conditionalizes the keyword of the variable depending on if there is a test or not.

# Result
The code is not generating any warnings any more.

Author: FranzBusch

Fixtures/Miscellaneous/TestDiscovery/NoTests/Package.swift

@@ -0,0 +1,10 @@
+// swift-tools-version:4.2
+import PackageDescription
+
+let package = Package(
+    name: "Simple",
+    targets: [
+        .target(name: "Simple"),
+        .testTarget(name: "SimpleTests", dependencies: ["Simple"]),
+    ]
+)

Fixtures/Miscellaneous/TestDiscovery/NoTests/Sources/Simple/Simple.swift

@@ -0,0 +1,3 @@
+struct Simple {
+
+}

Fixtures/Miscellaneous/TestDiscovery/NoTests/Tests/SimpleTests/SwiftTests.swift

@@ -0,0 +1 @@
+

Sources/Build/BuildOperationBuildSystemDelegateHandler.swift

@@ -151,6 +151,8 @@ final class TestDiscoveryCommand: CustomLLBuildCommand {
             throw InternalError("main output (\(LLBuildManifest.TestDiscoveryTool.mainFileName)) not found")
         }
 
+        let testsKeyword = tests.isEmpty ? "let" : "var"
+
         // Write the main file.
         let stream = try LocalFileOutputByteStream(mainFile)
 
@@ -160,7 +162,7 @@ final class TestDiscoveryCommand: CustomLLBuildCommand {
         stream <<< "@available(*, deprecated, message: \"Not actually deprecated. Marked as deprecated to allow inclusion of deprecated tests (which test deprecated functionality) without warnings\")" <<< "\n"
         stream <<< "struct Runner" <<< " {" <<< "\n"
         stream <<< indent(4) <<< "static func main()" <<< " {" <<< "\n"
-        stream <<< indent(8) <<< "var tests = [XCTestCaseEntry]()" <<< "\n"
+        stream <<< indent(8) <<< "\(testsKeyword) tests = [XCTestCaseEntry]()" <<< "\n"
         for module in testsByModule.keys {
             stream <<< indent(8) <<< "tests += __\(module)__allTests()" <<< "\n"
         }

Tests/FunctionalTests/TestDiscoveryTests.swift

@@ -54,6 +54,21 @@ class TestDiscoveryTests: XCTestCase {
         }
     }
 
+    func testDiscovery_whenNoTests() throws {
+        #if os(macOS)
+        try XCTSkipIf(true)
+        #endif
+        try fixture(name: "Miscellaneous/TestDiscovery/NoTests") { fixturePath in
+            let (stdout, stderr) = try executeSwiftTest(fixturePath)
+            // in "swift test" build output goes to stderr
+            XCTAssertMatch(stderr, .contains("Build complete!"))
+            // we are expecting that no warning is produced
+            XCTAssertNoMatch(stderr, .contains("warning:"))
+            // in "swift test" test output goes to stdout
+            XCTAssertMatch(stdout, .contains("Executed 0 tests"))
+        }
+    }
+
     func testManifestOverride() throws {
         #if os(macOS)
         try XCTSkipIf(true)