gdb: testsuite: show print array-indexes after set in arrayidx.exp

Add "show print array-indexes" testcases after set print array-indexes
to off or on.

Without this patch:

    PASS: gdb.base/arrayidx.exp: set print array-indexes to off
    PASS: gdb.base/arrayidx.exp: print array with array-indexes off
    PASS: gdb.base/arrayidx.exp: set print array-indexes to on
    PASS: gdb.base/arrayidx.exp: print array with array-indexes on

With this patch:

    PASS: gdb.base/arrayidx.exp: set print array-indexes to off
    PASS: gdb.base/arrayidx.exp: show print array-indexes is off
    PASS: gdb.base/arrayidx.exp: print array with array-indexes off
    PASS: gdb.base/arrayidx.exp: set print array-indexes to on
    PASS: gdb.base/arrayidx.exp: show print array-indexes is on
    PASS: gdb.base/arrayidx.exp: print array with array-indexes on

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>