tests: work around Tru64 sh -e issues for instspc*.test.

tests: work around Tru64 sh -e issues for instspc*.test.

Apparently, Tru64 sh does not like turning off errexit mode,
and gets confused.

* tests/defs: Document 'errexit' cleanliness requirement.
* tests/defs-static.in: Likewise.  Avoid error from command
substitution to abort instspc*.test with Tru64/OSF 5.1 sh.
* tests/instspc-tests.sh: Drop now-unneeded temporary errexit
dropping.  Add strategic '|| Exit' to let tests work on Tru64.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>