Replace the magic number 42 in PAssert.

[kennknowles]

PAssert used integer 42 as the global key. It causes some confusion when debugging ValidatesRunner tests.

final int combinedKey = 42;
https://github.com/apache/beam/find/master#L935

Imported from Jira BEAM-1854. Original Jira may contain additional context.
Reported by: peihe0@gmail.com.