File-Glob/t/basic.t: avoid race condition

Test 2 does a glob('*') in the t/ directory, and compares that with the
results of readdir('.'). If we're doing parallel testing, temporary files
and stuff may get created in t/, resulting in a race condition and
occasional random failures.

Fix this by chdir()ing to t/base/ first. The timestamp on this directory
on my system seems to indicate that nothing currently creates tmp files in
that dir, and given that these are basic tests, that's relatively unlikely
to change.