Hook up oss-fuzz test cases as tests

This is a bit painful because a separate build of systemd is necessary. The
tests are guarded by tests!=false and slow-tests==true. Running them is not
slow, but compilation certainly is. If this proves unwieldy, we can add a
separate option controlling those builds later.

The build for each sanitizer has its own directory, and we build all fuzzer
tests there, and then pull them out one-by-one by linking into the target
position as necessary. It would be nicer to just build the desired fuzzer, but
we need to build the whole nested build as one unit.

[I also tried making systemd and nested meson subproject. This would work
nicely, but meson does not allow that because the nested target names are the
same as the outer project names. If that is ever fixed, that would be the way
to go.]

v2:
- make sure things still work if memory sanitizer is not available
v3:
- switch to syntax which works with meson 0.42.1 found in Ubuntu

test/fuzz-regressions/meson.build

@@ -0,0 +1,30 @@
+# SPDX-License-Identifier: LGPL-2.1+
+#
+# Copyright 2018 Zbigniew Jędrzejewski-Szmek
+#
+# systemd is free software; you can redistribute it and/or modify it
+# under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation; either version 2.1 of the License, or
+# (at your option) any later version.
+#
+# systemd is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with systemd; If not, see <http://www.gnu.org/licenses/>.
+
+sanitize_address = custom_target(
+        'sanitize-address-fuzzers',
+        output : 'sanitize-address-fuzzers',
+        command : [meson_build_sh,
+                   meson.source_root(),
+                   '@OUTPUT@',
+                   'fuzzers',
+                   '-Db_lundef=false -Db_sanitize=address'])
+
+fuzz_regression_tests = '''
+        address/fuzz-dns-packet/oss-fuzz-5465
+        address/fuzz-dns-packet/issue-7888
+'''.split()

test/meson.build

@@ -228,3 +228,5 @@ if conf.get('ENABLE_HWDB') == 1
              hwdb_test_sh,
              timeout : 90)
 endif
+
+subdir('fuzz-regressions')