Python 3: port some tests in common and infrastructure (#24482)

diff --git a/common/echo.py b/common/echo.py
index 2ee4036..911b54a 100644
--- a/common/echo.py
+++ b/common/echo.py
@@ -1,6 +1,6 @@
 def main(request, response):
     # Without X-XSS-Protection to disable non-standard XSS protection the functionality this
     # resource offers is useless
-    response.headers.set("X-XSS-Protection", "0")
-    response.headers.set("Content-Type", "text/html")
-    response.content = request.GET.first("content")
+    response.headers.set(b"X-XSS-Protection", b"0")
+    response.headers.set(b"Content-Type", b"text/html")
+    response.content = request.GET.first(b"content")
diff --git a/common/slow.py b/common/slow.py
index f3b1c7e..5329a04 100644
--- a/common/slow.py
+++ b/common/slow.py
@@ -2,4 +2,4 @@
 
 def main(request, response):
     time.sleep(2)
-    return 200, [], ''
+    return 200, [], b''
diff --git a/infrastructure/testdriver/file_upload.py b/infrastructure/testdriver/file_upload.py
index 89ed67a..9c4dd75 100644
--- a/infrastructure/testdriver/file_upload.py
+++ b/infrastructure/testdriver/file_upload.py
@@ -1,2 +1,2 @@
 def main(request, response):
-    return "PASS" if request.POST[b"file_input"].file.read() == b"File to upload\n" else "FAIL"
+    return b"PASS" if request.POST[b"file_input"].file.read() == b"File to upload\n" else b"FAIL"