[gdb/testsuite] Make gdb.threads/inf-thr-count.exp more readable

[gdb/testsuite] Make gdb.threads/inf-thr-count.exp more readable

While investigating a timeout in gdb.threads/inf-thr-count.exp I noticed that
it uses quite some escaping, resulting in hard-to-parse regexps like
"\\\$$::decimal".

Fix this by reducing the escaping using:
- quoting strings using {} instead of "", and
- string_to_regexp.

Also use multi_line to split up long multi-line regexps.

Tested on x86_64-linux.