build: meson support for unit tests

Add support for building and running existing unit tests
under meson.

Fix a few test case failures that might surface from running
as non-root or without a full OpenBMC set of applications.

Fixes: openbmc/phosphor-logging#11

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I588c0b7d8dcdc60b9a03fee0b5d373d0023b0530
diff --git a/meson_options.txt b/meson_options.txt
index 5f3bd2f..de37d35 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -1,3 +1,5 @@
+option('tests', type: 'feature', description: 'Build tests')
+
 option('yamldir', type: 'string', description: 'Path to YAML')
 option(
     'callout_yaml',