gdb/testsuite: Remove duplicates from gdb.base/miscexprs.exp

When running the testsuite I see:

    Running .../gdb/testsuite/gdb.base/miscexprs.exp ...
    DUPLICATE: gdb.base/miscexprs.exp: print value of !ibig.i[100]
    DUPLICATE: gdb.base/miscexprs.exp: print value of !ibig.i[100]

This is due to an explicit test name repeated across multiple tests.
The actual test explicit names do not add much over the command from
wich default test names are derived.

Fix by removing the explicit test names across the file where they do
not add value.  While at doing some cleaning, also use $gdb_test_name in
the various uses of gdb_test_multiple.

Tested on x86_64-linux.