Make gdb.base/index-cache.exp work with readnow board (PR 24669)

Make gdb.base/index-cache.exp work with readnow board (PR 24669)

The gdb.base/index-cache.exp test fails with the readnow board:

  $ make check TESTS="gdb.base/index-cache.exp" RUNTESTFLAGS="--target_board=readnow"
  FAIL: gdb.base/index-cache.exp: test_cache_enabled_miss: at least one file was created
  FAIL: gdb.base/index-cache.exp: test_cache_enabled_miss: expected file is there
  FAIL: gdb.base/index-cache.exp: test_cache_enabled_miss: check index-cache stats
  FAIL: gdb.base/index-cache.exp: test_cache_enabled_hit: check index-cache stats

The problem is similar to what was fixed in

  5a56d6a65f84 ("[gdb/testsuite] Fix index-cache.exp with cc-with-{gdb-index,debug-names}")

In that commit, gdb.base/index-cache.exp was modified to account for the
fact that the index cache is not used when the binary already has an
embedded index.

The same situation happens when GDB is started with the -readnow flag:
it bypasses indices and partial symbols.  So this patch updates the test
to also expect the index cache not to be used if -readnow is present in
$GDBFLAGS,

gdb/testsuite/ChangeLog:

	PR gdb/24669
	* gdb.base/index-cache.exp (uses_readnow,
	expecting_index_cache_use): Define global variable.
	(test_cache_enabled_miss, test_cache_enabled_hit): Use
	expecting_index_cache_use.