[gdb/testsuite] Handle readnow in ensure_gdb_index

[gdb/testsuite] Handle readnow in ensure_gdb_index

When running test-case gdb.base/with-mf.exp with target board readnow, I run
into:
...
FAIL: gdb.base/with-mf.exp: check if index present
...
This is since commit 6010fb0c49e "[gdb/testsuite] Fix full buffer in
gdb.rust/dwindex.exp".

Before that commit, the proc ensure_gdb_index would treat the line:
...
.gdb_index: faked for "readnow"^M
...
as proof that an index is already present (which is incorrect).

Now, instead it generates aforementioned FAIL and continues to generate an
index.

Fix this by explicitly handling the readnow case in proc ensure_gdb_index,
such that we bail out instead.

Tested on x86_64-linux.