Enable compiling mjsunit tests as ES6 modules

Enable compiling mjsunit tests as ES6 modules

Adding the line "// MODULE" to an mjsunit file will now cause
run-tests.py to prefix the test case with "--module" in the
d8 commandline.

d8 has itself been updated to treat files preceded with "--module" as
modules (that is, it compiles them with ScriptCompiler::CompileModule,
and turns on --harmony-modules).

Review URL: https://codereview.chromium.org/902263002

Cr-Commit-Position: refs/heads/master@{#26555}