tests: optimise creating repositories and loading dumps Previously, we'd use svnadmin for creating repositories and loading dumps. That tends to be a bit slow, as it forks a new process and loads the Subversion libraries into it. Instead, we extend our existing Subversion wrappers and load the dumps using the API. This is a noticable speedup. The only downside is that we rely on Subversion and Subvertpy to correctly close all file descriptors; an assumption which hasn't always held in the past. I ran some benchmarks on my relatively slow Mac with $TMPDIR on a ramdisk, and they showed a significant change: I compared ten runs of each with Subvertpy: min: -18.8% (299.1s -> 243.0s) median: -20.0% (307.1s -> 245.6s) ...and three runs of each with SWIG: min: -22.8% (368.7s -> 284.7s) median: -25.7% (384.4s -> 285.5s) (Since the timing measures wall clock time, the minimum time is likely to be the most accurate and useful measurement.)