[CMAKE] Allow for installing test executables

Summary:
By default none is installed.

Installation via components (the usual way) is not really possible with
tests which have EXCLUDE_FROM_ALL targets. Depending on your cmake
version the support may vary, but generally speaking cmake doesn't know
how to handle it. A solution is to create a custom target to run the
installation that will take care of building the executable targets
first.

The tests can be installed via several install targets:

• install-tests builds and installs all the tests
• install-test-suite-<suite> builds and installs the tests from a

single test suite.

• install-<suite>-<test> builds an installs a single test from

Test Plan:
Note: no need to build anything prior running the commands.

ninja install-tests
ninja install-test-suite-avalanche
ninja install-leveldb-arena_test

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7209