Add SwiftDriverTests.testToolSearching test.

Author: stevapple

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -6395,6 +6395,48 @@ final class SwiftDriverTests: XCTestCase {
 #endif
 #endif
   }
+
+  func testToolSearching() throws {
+    var driver = try Driver(args: ["swiftc", "-print-target-info"])
+    let jobs = try driver.planBuild()
+    XCTAssertEqual(jobs.count, 1)
+    let defaultSwiftFrontend = jobs.first!.tool.absolutePath!
+    
+    try withTemporaryDirectory { toolsDirectory in
+      let customSwiftFrontend = toolsDirectory.appending(component: executableName("swift-frontend"))
+      try localFileSystem.createSymbolicLink(customSwiftFrontend, pointingAt: defaultSwiftFrontend, relative: false)
+
+      // test if PATH is searched
+      do {
+        var driver = try Driver(args: ["swiftc", "-print-target-info"], env: ["PATH": toolsDirectory.pathString])
+        let jobs = try driver.planBuild()
+        XCTAssertEqual(jobs.count, 1)
+        XCTAssertEqual(jobs.first!.tool.name, customSwiftFrontend.pathString)
+      }
+
+      try withTemporaryDirectory { tempDirectory in 
+        try localFileSystem.changeCurrentWorkingDirectory(to: tempDirectory)
+        let anotherSwiftFrontend = localFileSystem.currentWorkingDirectory!.appending(component: executableName("swift-frontend"))
+        try localFileSystem.createSymbolicLink(anotherSwiftFrontend, pointingAt: defaultSwiftFrontend, relative: false)
+
+        // test if tools directory is respected
+        do {
+          var driver = try Driver(args: ["swiftc", "-print-target-info", "-tools-directory", toolsDirectory.pathString])
+          let jobs = try driver.planBuild()
+          XCTAssertEqual(jobs.count, 1)
+          XCTAssertEqual(jobs.first!.tool.name, customSwiftFrontend.pathString)
+        }
+      
+        // test if current working directory is searched
+        do {
+          var driver = try Driver(args: ["swiftc", "-print-target-info"], env: ["PATH": ""])
+          let jobs = try driver.planBuild()
+          XCTAssertEqual(jobs.count, 1)
+          XCTAssertEqual(jobs.first!.tool.name, anotherSwiftFrontend.pathString)
+        }
+      }
+    }  
+  }
 }
 
 func assertString(