gdb/testsuite: Remove duplicates from gdb.base/shlib-call.exp

When I run the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/shlib-call.exp ...
    DUPLICATE: gdb.base/shlib-call.exp: print g
    DUPLICATE: gdb.base/shlib-call.exp: set print sevenbit-strings
    DUPLICATE: gdb.base/shlib-call.exp: set print address off
    DUPLICATE: gdb.base/shlib-call.exp: set width 0
    DUPLICATE: gdb.base/shlib-call.exp: continue until exit

Fix by adjusting the test names when required, and by removing
un-necessary commands.

While at it, do some cleanup:
- Replace an explicit GDB restart sequence with a call to clean_restart.
- Remove trailing whitespaces.
- Use $gdb_test_name in gdb_test_multiple.

Tested on x86_64-linux.