Add support for TEST_METADATA

Allow Makefiles to define generic metadata settings that apply to all test
programs defined by a Makefile. The generic TEST_METADATA variable extends
the per-test program settings already supported via TEST_METADATA.<program>.

This feature will be useful to easily apply some settings to all programs
in a directory. In particular, Kyua 0.12 will support parallel execution
of test programs and a bunch of them will need to be tagged as is_exclusive
to indicate that they cannot be run in parallel with anything else due to
their side-effects. It will be reasonable to set this setting on whole
directories.

MFC after: 1 week