configure-sampledata: only create random paths if asked

configure-sampledata: only create random paths if asked

Stop creating randomized paths in t/sampledata/ by default.  I'd
originally just added this to allow some quick testing, and while it
now appears to be fine on Linux/ext4, it's too aggressive to be the
default, so hide it behind a BUP_TEST_RANDOMIZED_SAMPLEDATA_PATHS
environment variable.

Among other things, make-random-paths just crashes on cirrus macos,
and cirrus freebsd was having (different) trouble.  It might also have
been macos where test-import-duplicity.sh failed on compare-trees
mismatches.  Not sure whether that was an issue with bup, rsync, or
duplicity.

We'll want to restore broader randomized path testing, but likely via
a less blunt instrument, since placing the paths in t/sampledata
affects any test that relies on it, and existing testing constructs
like

  WVPASSEQ ... $(... | wc -l)

are completely incompatible.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>