Replace shared variables for test http server with structure fields

[gboyvalenkov-bosch]

Currently there are 3 shared variables in http_test_utl.go: failCountBadStatus, failCopyError, corruptFileError. They should be replaced with fields within TestHTTPServer structure, in order to make code more readable and understandable. However this requires revision of http handler functions. Currently only 1 test alias is used for all methods, which implies that handler function is registered only once.
Since this function is a pointer receiver of TestHTTPServer, it uses the fields from the first created instance, which makes it impossible to modify behavior.