build: avoid non-portable shell in test setup

build: avoid non-portable shell in test setup

POSIX states that 'a=1; a=2 b=$a command' has unspecified results
for the value of $b visible within command.  In particular, on
BSD, this resulted in PATH not picking up the in-test ssh.

* tests/Makefile.am (lv_abs_top_builddir): New macro.
(path_add, TESTS_ENVIRONMENT): Use it to avoid referring to an
environment variable set previously within the same command line.
Reported by Matthias Bolte.