Only build regulator tests when option enabled

Currently the regulator tests are always built.  This causes problems in
the Witherspoon SDK because it seems to contain an older, incompatible
version of gmock.

Modified the regulators meson file to only build the tests if the meson
'tests' option is enabled.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I5bfe6ee22487085d37832e4b7b0d9f63c8751c0a
diff --git a/phosphor-regulators/meson.build b/phosphor-regulators/meson.build
index 774868c..82a23ee 100644
--- a/phosphor-regulators/meson.build
+++ b/phosphor-regulators/meson.build
@@ -1,2 +1,5 @@
 subdir('src')
-subdir('test')
+
+if get_option('tests').enabled()
+    subdir('test')
+endif