Add an expectation of a warning diagnostic to tests that hide libSwiftScan.

Such tests cause the driver to take a code-path that results in a warning:
'warning: Unable to locate libSwiftScan. Fallback to `swift-frontend` dependency scanner invocation.'

This needs to be taken into account.

Author: artemcm

Tests/CommandsTests/PackageCommandTests.swift

@@ -2267,6 +2267,8 @@ final class PackageCommandTests: CommandsTestCase {
         let containsRemark = StringPattern.contains("command plugin: Diagnostics.remark")
         let containsWarning = StringPattern.contains("command plugin: Diagnostics.warning")
         let containsError = StringPattern.contains("command plugin: Diagnostics.error")
+        let hiddenLibSwiftScanWarnPattern = StringPattern.equal(
+          "warning: \'miscellaneous_plugins_commandpluginteststub\': warning: Unable to locate libSwiftScan. Fallback to `swift-frontend` dependency scanner invocation.\nwarning: Unable to locate libSwiftScan. Fallback to `swift-frontend` dependency scanner invocation.\nwarning: Unable to locate libSwiftScan. Fallback to `swift-frontend` dependency scanner invocation.")
 
         try await fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
             func runPlugin(flags: [String], diagnostics: [String], completion: (String, String) -> Void) async throws {
@@ -2290,7 +2292,7 @@ final class PackageCommandTests: CommandsTestCase {
 
             try await runPlugin(flags: [], diagnostics: ["print"]) { stdout, stderr in
                 XCTAssertMatch(stdout, isOnlyPrint)
-                XCTAssertMatch(stderr, isEmpty)
+                XCTAssertMatch(stderr, hiddenLibSwiftScanWarnPattern)
             }
 
             try await runPlugin(flags: [], diagnostics: ["print", "progress"]) { stdout, stderr in
@@ -2322,7 +2324,7 @@ final class PackageCommandTests: CommandsTestCase {
 
             try await runPlugin(flags: ["-q"], diagnostics: ["print"]) { stdout, stderr in
                 XCTAssertMatch(stdout, isOnlyPrint)
-                XCTAssertMatch(stderr, isEmpty)
+                XCTAssertMatch(stderr, hiddenLibSwiftScanWarnPattern)
             }
 
             try await runPlugin(flags: ["-q"], diagnostics: ["print", "progress"]) { stdout, stderr in
@@ -2461,7 +2463,11 @@ final class PackageCommandTests: CommandsTestCase {
         let containsLogtext = StringPattern.contains("command plugin: packageManager.build logtext: Building for debugging...")
 
         // Echoed logs have no prefix
-        let containsLogecho = StringPattern.regex("^Building for debugging...\n")
+        let containsLogecho = StringPattern.contains("Building for debugging...\n")
+
+        // Hides the libSwiftScan library from the driver
+        let hiddenLibSwiftScanWarnPattern = StringPattern.equal(
+          "warning: \'miscellaneous_plugins_commandpluginteststub\': warning: Unable to locate libSwiftScan. Fallback to `swift-frontend` dependency scanner invocation.\nwarning: Unable to locate libSwiftScan. Fallback to `swift-frontend` dependency scanner invocation.\nwarning: Unable to locate libSwiftScan. Fallback to `swift-frontend` dependency scanner invocation.")
 
         // These tests involve building a target, so each test must run with a fresh copy of the fixture
         // otherwise the logs may be different in subsequent tests.
@@ -2470,14 +2476,14 @@ final class PackageCommandTests: CommandsTestCase {
         try await fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
             let (stdout, stderr) = try await SwiftPM.Package.execute(["print-diagnostics", "build"], packagePath: fixturePath, env: ["SWIFT_DRIVER_SWIFTSCAN_LIB" : "/this/is/a/bad/path"])
             XCTAssertMatch(stdout, isEmpty)
-            XCTAssertMatch(stderr, isEmpty)
+            XCTAssertMatch(stderr, hiddenLibSwiftScanWarnPattern)
         }
 
         // Check that logs are returned to the plugin when echoLogs is false
         try await fixture(name: "Miscellaneous/Plugins/CommandPluginTestStub") { fixturePath in
             let (stdout, stderr) = try await SwiftPM.Package.execute(["print-diagnostics", "build", "printlogs"], packagePath: fixturePath, env: ["SWIFT_DRIVER_SWIFTSCAN_LIB" : "/this/is/a/bad/path"])
             XCTAssertMatch(stdout, containsLogtext)
-            XCTAssertMatch(stderr, isEmpty)
+            XCTAssertMatch(stderr, hiddenLibSwiftScanWarnPattern)
         }
 
         // Check that logs echoed to the console (on stderr) when echoLogs is true