acc: simple conditional error injection (#3437)

## Why
To test deployment when some of the resources fail to deploy. Static
Server block is not enough, since error is needed on some resources but
not others.

## Tests
New self test.

Author: denik

acceptance/selftest/inject_error/out.test.toml

@@ -0,0 +1,5 @@
+Local = true
+Cloud = false
+
+[EnvMatrix]
+  DATABRICKS_CLI_DEPLOYMENT = ["terraform", "direct-exp"]

acceptance/selftest/inject_error/output.txt

@@ -0,0 +1,5 @@
+
+>>> musterr [CLI] schemas create schema-INJECT_ERROR main
+Error: INJECTED
+
+Exit code (musterr): 1

acceptance/selftest/inject_error/script

@@ -0,0 +1,2 @@
+# If INJECT_ERROR is present anywhere in the request, test server will reply with 500 error
+trace musterr $CLI schemas create schema-INJECT_ERROR main

libs/testserver/server.go

@@ -1,6 +1,7 @@
 package testserver
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -261,8 +262,17 @@ func (s *Server) Handle(method, path string, handler HandlerFunc) {
 			s.RequestCallback(&request)
 		}
 
-		respAny := handler(request)
-		resp := normalizeResponse(s.t, respAny)
+		var resp EncodedResponse
+
+		if bytes.Contains(request.Body, []byte("INJECT_ERROR")) {
+			resp = EncodedResponse{
+				StatusCode: 500,
+				Body:       []byte("INJECTED"),
+			}
+		} else {
+			respAny := handler(request)
+			resp = normalizeResponse(s.t, respAny)
+		}
 
 		for k, v := range resp.Headers {
 			w.Header()[k] = v