test: use saved process PID instead of %%
    
    As the `%%` specifier might fail if the current job (i.e. the last
    background job) already finished:
    
    ```
    [   61.692196] testsuite-04.sh[656]: ++ systemd-id128 new
    [   61.705407] testsuite-04.sh[263]: + ID=912cb8f8ef304153a123f772bb0fe9e0
    [   61.706318] testsuite-04.sh[657]: + systemd-cat -t 912cb8f8ef304153a123f772bb0fe9e0 bash -c 'echo parent; (echo child) & wait'
    [   61.720940] testsuite-04.sh[263]: + PID=657
    [   61.721126] testsuite-04.sh[263]: + wait %%
    [   61.723014] testsuite-04.sh[263]: /usr/lib/systemd/tests/testdata/units/testsuite-04.sh: line 96: wait: %%: no such job
    ```
    
    (cherry picked from commit 08970485003c25ce2c4adfaeea2d58558d311d42)