Explicitly set dynamic variable buffer size.

We added too many variable assignments in BEGIN block, which will run out of
default auto-configured variable buffer space.  The test VM has 4G RAM which
should be enough for most cases so it's reasonable to increase limitation to
these case.

Reviewed by:	gnn, markj, ngie
Differential Revision:	https://reviews.freebsd.org/D11674

cddl/contrib/opensolaris/cmd/dtrace/test/tst/common/funcs/tst.cleanpath.d

@@ -27,6 +27,7 @@
 #pragma ident	"%Z%%M%	%I%	%E% SMI"
 
 #pragma D option quiet
+#pragma D option dynvarsize=2m
 
 BEGIN
 {