rustpkg: Un-ignore most of the remaining tests

This necessitated some cleanup to how we parse library filenames
when searching for libraries, since rustpkg may now create filenames
that contain '-' characters. Also cleaned up how rustpkg passes the
sysroot to a custom build script.
auto merge of #8548 : catamorphism/rust/unignore-rustpkg-tests, r=brson

r? @brson This necessitated some cleanup to how we parse library filenames
when searching for libraries, since rustpkg may now create filenames
that contain '-' characters. Also cleaned up how rustpkg passes the
sysroot to a custom build script.