Use /nonexistent as destination for files not to be installed.

Use /nonexistent as destination for files not to be installed.

* tests/darwin.at (darwin fat compile): Fix typo.
* tests/inherited_flags.at (inherited_linker_flags): Change
-rpath to /nonexistent.  This helps to avoid accidentally
picking up libraries below /usr/local.
* tests/template.at (simple template test): Likewise.  Fixes
test failure for additional incompatible libstdc++ in
/usr/local.
* tests/testsuite.at (configure_options): Add
--prefix=/nonexistent.

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