Introduce pg_receivewal gzip compression tests

Hi,

As suggested on a different thread [1], pg_receivewal can increase it's test
coverage. There exists a non trivial amount of code that handles gzip
compression. The current patch introduces tests that cover creation of gzip
compressed WAL files and the handling of gzip partial segments. Finally the
integrity of the compressed files is verified.

I hope you find this useful.

Cheers,
//Georgios

[1]
https://www.postgresql.org/message-id/flat/ZCm1J5vfyQ2E6dYvXz8si39HQ2gwxSZ3IpYaVgYa3lUwY88SLapx9EEnOf5uEwrddhx2twG7zYKjVeuP5MwZXCNPybtsGouDsAD1o2L_I5E%3D%40pm.me