Do not assume that locate command is available

[danielhollas]

When trying to find the pg_ctl binary, this package uses the locate command as a fallback when the command is not found in $PATH.

pgtest/pgtest/pgtest.py

Line 117 in 417f0d8

locate_cmd = ['locate', '-r', exact_file_regex]

However, locate command itself might not be available, for example in Docker containers. In this case, the package crashes with rather unhelpful stacktrace.

[ltalirz]

That's a great point. A pull request that adds a clean try/except for the availability of locate would be much appreciated