meson: Add googletest as a subproject

Allow the tests to build without requiring a system-wide installation of
googletest.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Iaf685cd1886672a435ba66c4aae3a0772b714dd5
diff --git a/subprojects/googletest.wrap b/subprojects/googletest.wrap
new file mode 100644
index 0000000..56da9ef
--- /dev/null
+++ b/subprojects/googletest.wrap
@@ -0,0 +1,3 @@
+[wrap-git]
+url = https://github.com/google/googletest
+revision = HEAD