Address #1070 @everythingfunctional.

This PR proposes to add the option to also install the test programs in the installation directory.
Two methods are made possible, similar to what can be done for a library install:

1. Via manifest

User can now specify a test boolean key in fpm.toml to request that test programs are installed as well

[install]
library = true
test = true

2. Via CLI

User can request tests to be installed via the --test flag.

The default installation subdirectory is test/, but the user can request a custom subdirectory via --testdir "testdir".
In both cases, this is a subdirectory created within the installation $PREFIX.

fpm install --prefix=/path --test --testdir "my_test_dir"

Full description is reported in fpm help install.

cc: @everythingfunctional @urbanjost @fortran-lang/fpm