[WIP] Weak-link to XCTest

Author: grynspan

Sources/Build/LLBuildCommands.swift

@@ -259,7 +259,7 @@ final class TestEntryPointCommand: CustomLLBuildCommand, TestBuildCommand {
             #if \#(isXCTMainAvailable)
             \#(generateTestObservationCode(buildParameters: buildParameters))
 
-            import XCTest
+            @_weakLinked import XCTest
             \#(discoveryModuleNames.map { "import \($0)" }.joined(separator: "\n"))
             #endif
 
@@ -298,7 +298,7 @@ final class TestEntryPointCommand: CustomLLBuildCommand, TestBuildCommand {
                     }
                     #endif
                     #if \#(isXCTMainAvailable)
-                    if testingLibrary == "xctest" {
+                    if testingLibrary == "xctest", #_hasSymbol(XCTMain) {
                         \#(testObservabilitySetup)
                         \#(awaitXCTMainKeyword) XCTMain(__allDiscoveredTests()) as Never
                     }