Item test should limit the returned value to a certain amount of data

After implementation of ZBXNEXT-6554 some items that use external commands or return file contents ( line vfs.file.contents, system.sw.packaget and system.sw.packages.get) return up to 16 Mb of data (system.sw.packages.get - up to 32 Mb).
Currently Item test functionality returns the whole value that is returned by the metric, which is then used to test all of the preprocessing steps. In case of such huge values this will drain PHP allocated memory which will eventually result in internal system error (500 error).

The amount of data that is retrieved by item test should be limited to a value that will not cause such memory issues (like 512 Kb for example).