[gdb/testsuite] Fix check-read1 FAIL with gdb.base/maint.exp

When running test-case gdb.base/maint.exp with check-read1, I run into:
...
FAIL: gdb.base/maint.exp: (timeout) maint print objfiles
...

The FAIL happens because command output contains long lines like this:
...
file1 at $hex, file2 at $hex, ..., $file$n at $hex,
...
F.i., such a line for libc.so.debug contains 82000 chars.

Fix this this by reading long lines bit by bit.

Also, replace the testing of the command output formulated using a gdb_send
combined with gdb_expect-in-a-loop, with a regular gdb_test_multiple with
exp_continue.

Tested on x86_64-linux, with make targets check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-15  Tom de Vries  <tdevries@suse.de>

	* gdb.base/maint.exp: Use exp_continue in long lines for "maint print
	objfiles".