Adapt test to work better for MaD

In MaD, `Argument[n]` corresponds to the post-update node of the
argument, which in the old version of the test will be the definition of
`dstReader` for the tests for `ReadBody`, `ReadLimitBody`,
`ContinueReadBodyStream`, `ContinueReadBody`.

Author: owen-mc

go/ql/test/library-tests/semmle/go/frameworks/Fasthttp/fasthttp.go

@@ -169,10 +169,10 @@ func fasthttpServer() {
 		fmt.Println(body1, body2, body3, body4)
 		requestCtx.Request.BodyStream() // $ RemoteFlowSource="call to BodyStream"
 
-		requestCtx.Request.ReadBody(dstReader, 100, 1000)               // $ RemoteFlowSource="dstReader"
-		requestCtx.Request.ReadLimitBody(dstReader, 100)                // $ RemoteFlowSource="dstReader"
-		requestCtx.Request.ContinueReadBodyStream(dstReader, 100, true) // $ RemoteFlowSource="dstReader"
-		requestCtx.Request.ContinueReadBody(dstReader, 100)             // $ RemoteFlowSource="dstReader"
+		requestCtx.Request.ReadBody(&bufio.Reader{}, 100, 1000)               // $ RemoteFlowSource="&..."
+		requestCtx.Request.ReadLimitBody(&bufio.Reader{}, 100)                // $ RemoteFlowSource="&..."
+		requestCtx.Request.ContinueReadBodyStream(&bufio.Reader{}, 100, true) // $ RemoteFlowSource="&..."
+		requestCtx.Request.ContinueReadBody(&bufio.Reader{}, 100)             // $ RemoteFlowSource="&..."
 
 		// Response methods
 		// Xss Sinks Related method