Test suite: Provide t-setup-* machinery

Setups are, essentially, prefixes to test cases.  We arrange to be
able to reuse their state (as explicitly recorded in t-setup-done), so
that manual runs of the test suite can be faster.

We save the tmp base directory (the directory shared by tests), if
there is one.  That will be used for cacheing of setups.  And we save
the environment in form we can use xargs and env to restore.

We use a conventional with-lock-ex and stamp file pattern for the
cache.  The IMPORT file (containing shell variables) doubles as the
stamp.

The setup execution rune is exciting.  This mostly results from the
fact that we want xargs to pass all the env vars to env, and that
xargs cannot be persuaded to append fixed arguments to the arguments
read from its input file.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>