Use test install base template instead of a hardcoded path

Make the template more consistent with the native (aka cc) test temlate.

The important change is that vendor tests will be pushed into
/data/local/tests/vendor. In fact, rust module in soong already expects
this (aosp/1832614).

Having the test binary in the vendor test folder makes the linker treat
it as a vendor binary and prefer vendor libraries. Previously, the test
binary was always a system binary (due to being in /data/local/tmp) and
used system libraries first potentially leading to bugs (e.g. the one
attached).

This change also makes vendor tests that use vendor libraries require
root due to an existing SELinux policy issue with atest.

Bug: 347370677
Test: checked with:
 - libsecretkeeper_core_test
 - libauthgraph_boringssl_test (with the chained change)
 - libdice_policy.test
 Test steps:
 atest -d <test>.vendor # runs vendor variant, passes
 adb root
 adb shell find / -iname <test> 2>/dev/null
 # before change: /data/local/tmp/<test>
 # after change: /data/local/tests/vendor/<test>
Change-Id: I6d8c58cbce5b636480803e74cb710f0b05b74e10