[gdb/testsuite] Fix gdb.base/info_sources_2.exp with check-read1

[gdb/testsuite] Fix gdb.base/info_sources_2.exp with check-read1

When running test-case gdb.base/info_sources_2.exp with check-read1, I run
into:
...
FAIL: gdb.base/info_sources_2.exp: args: : info sources  (timeout)
...

Fix this by consuming a "$src1, $src2, ..., $srcn: line bit by bit rather than
as one whole line.

Also add the missing handling of "Objfile has no debug information".

Tested on x86_64-linux.