Move conditional compilation directive inside tests.

This ensures that the test function always exists, even if the
symbol is not defined, so that `--generate-linuxmain` contains
the symbol in both cases. When the symbol is undefined, the
empty test will vacuously succeed.

Author: allevato

Tests/SwiftFormatPrettyPrintTests/XCTestManifests.swift

@@ -207,7 +207,9 @@ extension DifferentiationAttributeTests {
     //   `swift test --generate-linuxmain`
     // to regenerate.
     static let __allTests__DifferentiationAttributeTests = [
+        ("testDerivative", testDerivative),
         ("testDifferentiable", testDifferentiable),
+        ("testTranspose", testTranspose),
     ]
 }