[gdb/testsuite] Use valnum_re more often (part 4)

Grepping through the testsuite I also found:
- "\\$\[0-9\]\+"
- "\\$\[0-9\]\*"
- "\\$\[0-9\]".

Also replace these with valnum_re.

Approved-by: Kevin Buettner <kevinb@redhat.com>