testsuite: Isolate aclocal from third-party macros (#110352).

testsuite: Isolate aclocal from third-party macros (#110352).

Several tests in the testsuite run a system-provided aclocal, which
will look into its $prefix/share/aclocal for third-party macros.
If those macros are buggy, aclocal may bomb out even though the test
doesn’t use them, causing the test to fail spuriously.

In all tests that need to run aclocal, create an empty directory and
give aclocal the --system-acdir option pointing at that directory.
This masks out all these potentially buggy macros.  (It does *not*
mask out AM_INIT_AUTOMAKE, which aclocal will find in a different
directory.)

In all tests that run autoreconf but *don’t* need to run aclocal,
create an empty aclocal.m4 and set ACLOCAL=true in the environment.

Fixes bug #110352.  Problem reported by Dagobert Michelsen.

* tests/fortran.at
* tests/tools.at
* tests/torture.at:
  Set ACLOCAL=true in the environment in all tests that run
  autoreconf but don’t need to run aclocal.
  Set ACLOCAL="aclocal --system-acdir <empty directory>" in all
  tests that do need to run aclocal.