Frequent timeouts when waiting for mock dbus service to appear on the bus

I can't reproduce this locally, but the system-apps' jenkaas slaves frequently hit this issue.

What happens:
15:05:47 Config: Using QtTest library 5.6.1, Qt 5.6.1 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 5.4.0 20160609)
15:05:47 PASS : TstSystemImage::initTestCase()
15:05:47 FAIL! : TstSystemImage::testDetailedVersionDetails() Caught unhandled exception
15:05:47 Loc: [qtestcase.cpp(2982)]
15:05:47 Totals: 1 passed, 1 failed, 0 skipped, 0 blacklisted
15:05:47 ********* Finished testing of TstSystemImage *********
15:05:47 terminate called after throwing an instance of 'std::logic_error'
15:05:47 what(): Process [python3] for service [com.canonical.SystemImage] failed to appear on bus
15:05:47
15:05:47 Start 4: python3

What should happen:
It's passing 100% of the time locally, on the same arch and release (not illustrated by the above log), so I assume that it should also pass on jenkaas.

The server is under incredible load. A test running for, on average, < 0.5 secs locally can take 35 secs on jenkaas.