c++/modules testsuite: restrict expensive pr99023 test

c++/modules testsuite: restrict expensive pr99023 test

The pr99023 testcase uses --param=ggc-min-expand=0 which forces a GC
during every collection point and consequently is very slow to run,
and ends up being the main bottleneck of the modules.exp testsuite.

So this patch restricts this test to run once, in C++20 mode, instead of
multiple times (C++17, C++20 and C++23 mode by default).  After this
patch the modules.exp testsuite finishes in 3m instead of 3m40s with -j8
on my machine.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr99023_a.X: Run only in C++20 mode.
	* g++.dg/modules/pr99023_b.X: Likewise.

Reviewed-by: Jason Merrill <jason@redhat.com>