We were defining the E10S_TESTING_ONLY build-time define with the value of the E10S_TESTING_ONLY config variable,
regardless of the value of E10S_TESTING_ONLY. Even if E10S_TESTING_ONLY in configure was blank, we'd still define
E10S_TESTING_ONLY, which resulted in E10S_TESTING_ONLY ifdef'd code to be included when we didn't want it to.
We now only ever define E10S_TESTING_ONLY iff NIGHTLY_BUILD is defined, and we do it globally.
This also fixes a busted E10S_TESTING_ONLY ifdef that didn't have an endif.
Every directory with a jar.mn now has JAR_MANIFESTS defined in its
moz.build file.
We also removed the may_skip special consideration of jar.mn files
because this information is now available during tier traversal by the
reader courtesy of the variables being present in moz.build files.
