Enable test builds with --bootstrap

[am11]

• Enable UseBootstrap in libraries.tests
• Enable --use-bootstrap in runtime testss (src/tests/build.sh --use-bootstrap --cross --riscv64 ...)
• Use local target pack infra from eng/ in src/tests (once discussed with @hoyosjs Enable building tests with UseLocalAppHostPack=true #99354 (review))

Fixes #115622

[am11]

For community platforms, this is the cross-build workflow for runtime development/testing:

os=linux
arch=riscv64

# ROOTFS_DIR is set

# cross-build product, --bootstrap
./build.sh clr+libs --cross --arch $arch --os $os --bootstrap

# for rebuild (e.g. after editing code), --use-bootstrap
./build.sh clr+libs --cross --arch $arch --os $os --use-bootstrap

# runtime tests cross-build, --use-bootstrap
src/tests/build.sh -cross -$arch -$os -p:LibrariesConfiguration=Debug --use-bootstrap

# libraries tests cross-build, --use-bootstrap
./build.sh libs.tests --cross --arch $arch --os $os --use-bootstrap

cc:
riscv64: @clamp03, @dotnet/samsung
loongarch64: @LuckyXu-HF, @shushanhf
illumos: @gwr
FreeBSD: @Thefrank, @sec
Haiku: @trungnt2910
OpenBSD: @sethjackson

[jkotas]

Update PR title to match the content?

[am11]

Update PR title to match the content?

Ah, it somehow used cached title from my previous PR.

[Thefrank]

So if I am reading this correctly, after this PR, I can:

• Use Enable test builds with --bootstrap #127909 (comment) to make nightly runtime tests for freebsd-ARCH
• Copy over zipped tests to freebsd system (assuming /p:ArchiveTests still exists if not done automatically)
• Run
• ...and finally enjoy easy nightly builds

This is pretty awesome. I gave up trying to do nightly runtime builds / tests more than a year ago because it was such a hassle natively.