Use a temporary directory as the build directory during tests so files
don't get written to the source tree.
Also add a few more tests for prebuilts with no file specified.
Bug: 31800129
Test: m -j, make sure .soong.environment is not written to the source tree
Change-Id: I623bc114b2ff534c8df9fb3ce273e804711f8f05
Support prebuilt shared libraries as an initial proof-of-concept of
prebuilts. Future changes will support binaries and static libraries,
and the ability to select which to use based on something besides
blueprint properties.
Test: TestPrebuilts run during m -j
Change-Id: I6e84da667e9005ae11844bad01d25cbe4ced1ce3
