gnu: subversion: Disable parallel tests.

gnu: subversion: Disable parallel tests.

These seem to work fine on x86_64-linux, but on i686-linux some tests fail
when running in parallel.

FAIL:  fs-fs-pack-test 4: commit to a packed FSFS filesystem
FAIL:  fs-test: Unknown test failure; see tests.log.
FAIL:  io-test: Unknown test failure; see tests.log.
FAIL:  op-depth-test: Unknown test failure; see tests.log.
FAIL:  wc-test: Unknown test failure; see tests.log.
Summary of test results:
  2243 tests PASSED
  96 tests SKIPPED
  43 tests XFAILED
  5 tests FAILED
SUMMARY: Some tests failed.

* gnu/packages/version-control.scm (subversion)[arguments]: Add
 #:parallel-tests? #f and use this in the 'set-PARALLEL phase.