Change main file tests to define sources inline

The tests are structured quite differently now but I think the new, smaller test cases cover all the interesting cases from the old test.

Author: ahoppen

Sources/SKTestSupport/INPUTS/MainFiles/a.swift

@@ -113,11 +113,13 @@ public class MultiFileTestWorkspace {
   /// Opens the document with the given file name in the SourceKit-LSP server.
   ///
   /// - Returns: The URI for the opened document and the positions of the location markers.
-  public func openDocument(_ fileName: String) throws -> (uri: DocumentURI, positions: DocumentPositions) {
+  public func openDocument(_ fileName: String, language: Language? = nil) throws -> (
+    uri: DocumentURI, positions: DocumentPositions
+  ) {
     guard let fileData = self.fileData[fileName] else {
       throw Error.fileNotFound
     }
-    let positions = testClient.openDocument(fileData.markedText, uri: fileData.uri)
+    let positions = testClient.openDocument(fileData.markedText, uri: fileData.uri, language: language)
     return (fileData.uri, positions)
   }
 

Sources/SKTestSupport/INPUTS/MainFiles/b.swift

@@ -78,6 +78,10 @@ public class SwiftPMTestWorkspace: MultiFileTestWorkspace {
       "-Xswiftc", "-index-ignore-system-modules",
       "-Xcc", "-index-ignore-system-symbols",
     ]
-    try await Process.checkNonZeroExit(arguments: arguments)
+    var environment = ProcessEnv.vars
+    // FIXME: SwiftPM does not index-while-building on non-Darwin platforms for C-family files (rdar://117744039). 
+    // Force-enable index-while-building with the environment variable.
+    environment["SWIFTPM_ENABLE_CLANG_INDEX_STORE"] = "1"
+    try await Process.checkNonZeroExit(arguments: arguments, environment: environment)
   }
 }