concretizer: allow a bool to be passed as argument for tests dependencies (#20082)

refers #20079

Added docstrings to 'concretize' and 'concretized' to
document the format for tests.

Added tests for the activation of test dependencies.

var/spack/repos/builtin.mock/packages/a/package.py

@@ -31,6 +31,7 @@ class A(AutotoolsPackage):
     variant('bvv', default=True, description='The good old BV variant')
 
     depends_on('b', when='foobar=bar')
+    depends_on('test-dependency', type='test')
 
     parallel = False
 

var/spack/repos/builtin.mock/packages/b/package.py

@@ -13,3 +13,5 @@ class B(Package):
     url      = "http://www.example.com/b-1.0.tar.gz"
 
     version('1.0', '0123456789abcdef0123456789abcdef')
+
+    depends_on('test-dependency', type='test')

var/spack/repos/builtin.mock/packages/test-dependency/package.py

@@ -0,0 +1,12 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+class TestDependency(Package):
+    """Represent a dependency that is pulled-in to allow testing other
+    packages.
+    """
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/tdep-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')