[gdb/testsuite] Fix gdb.cp/nested-types.exp with check-read1

[gdb/testsuite] Fix gdb.cp/nested-types.exp with check-read1

With check-read1 I occasionally run into:
...
FAIL: gdb.cp/nested-types.exp: ptype S10 (limit = 7) \
  // parse failed (timeout)
...
I can trigger this reliably by running check-read1 in conjunction with
stress -c 5.

Fix this by breaking up the regexp in cp_test_ptype_class.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-06-10  Tom de Vries  <tdevries@suse.de>

	* lib/cp-support.exp (cp_test_ptype_class): Break up regexp.
	* gdb.cp/nested-types.exp: Remove usage of read1 timeout factor.