Always emit rules for tests and add phony to run them

Emit the rules to build blueprint tests even if runGoTests is not
set, and add a phony rule "blueprint_tests" to run them.  This will
allow Soong to stop running the tests at the beginning of every build
but still run them as part of checkbuild or with a manual
`m blueprint_tests`.

Bug: 156428456
Test: m
Test: m blueprint_tests
Change-Id: If293a0757766d3046e78bf230a1825f15adc68fd