meson: create a static version of libshared and link libcore and some tests to it

This is what autoconf-based build does, and it makes test-bus-error and
test-engine able to access the bus error mapping table. OTOH, this is a heavy
price to pay: it would be excellent to link libcore.a to libsystemd-shared-NNN.so.
Otherwise we duplicate the same code in 'systemd' and 'libsystemd-shared-NNN.so'.

-rwxrwxr-x. 1 4075544 Apr  6 20:30 systemd* <-- libcore linked against libsystemd-shared.so
-rwxrwxr-x. 1 5596504 Apr  9 14:07 systemd* <-- libcore linked against libsystemd-shared.a

v2:
- update for 6b5cf3ea62

src/libsystemd/meson.build

@@ -129,10 +129,6 @@ tests += [
    [],
    [threads]],
 
-  [['src/libsystemd/sd-bus/test-bus-error.c'],
-   [],
-   []],
-
   [['src/libsystemd/sd-bus/test-bus-gvariant.c'],
    [],
    [libglib,