Update BuildToolTests.testBuildCompleteMessage

Motivation:
The build output on Linux for Swift >= 5.5 has changed. Causing self hosted Linux test to fail ([example](https://ci.swift.org/job/swift-package-manager-self-hosted-Linux-smoke-test/2051/)):

```
21:29:49 Test Case 'BuildToolTests.testBuildCompleteMessage' started at 2021-05-13 04:29:06.495
21:29:49
/home/buildnode/jenkins/workspace/swift-package-manager-self-hosted-Linux-smoke-test/branch-main/swiftpm/Tests/CommandsTests/BuildToolTests.swift:256: error: BuildToolTests.testBuildCompleteMessage : XCTAssertTrue failed - [1/4] Compiling Bar Bar.swift
21:29:49 [3/6] Merging module Bar
21:29:49 [5/7] Wrapping AST for Bar for debugging
21:29:49 [6/9] Compiling Foo Foo.swift
21:29:49 [7/9] Compiling Foo main.swift
21:29:49 [9/11] Merging module Foo
21:29:49 [11/12] Wrapping AST for Foo for debugging
21:29:49 [12/12] Linking Foo
21:29:49 [12/12] Build complete!
21:29:49
21:29:49 Test Case 'BuildToolTests.testBuildCompleteMessage' failed (5.24 seconds)
```

Modification:
Update expected output in `BuildToolTests.testBuildCompleteMessage`.

Author: yim-lee

Tests/CommandsTests/BuildToolTests.swift

@@ -252,6 +252,8 @@ final class BuildToolTests: XCTestCase {
                 let result = try execute([], packagePath: path)
                 #if os(macOS)
                 XCTAssertTrue(result.stdout.contains("[6/6] Build complete!"), result.stdout)
+                #elseif compiler(>=5.5)
+                XCTAssertTrue(result.stdout.contains("[12/12] Build complete!"), result.stdout)
                 #else
                 XCTAssertTrue(result.stdout.contains("[8/8] Build complete!"), result.stdout)
                 #endif