go/internal/gcimporter: pass -p to compiler in test
go/internal/gcimporter: pass -p to compiler in test

An upcoming Go 1.19 change will make -p required
when invoking the compiler. Fix this test to add it.
(See CL 391014.)

Change-Id: I874e6e2c3a03e1368ccfe2c770a694824c254c5b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/391034
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools