Extract scraper test code into common package

[djaglowski]

The newly added receivers for httpd and mysql have essentially the same test code for validating an expected result.

This code may be useful elsewhere eventually, but it would seem there is an immediate benefit to pulling into into a common package. I suggest we start with a internal/scrapertest. Potentially, this can eventually move into core if it has proves to be broadly useful.

[djaglowski]

@Mrod1598