Fix bugs with serve vs emulators:start

[samtstern]

Changes:

• Fixes When serve --only hosting, functions is executed in 6.9.0, there is no choice but to invoke only functions #1242
• Makes firebase serve start both hosting and functions if they've both been initialized.
• Adds a connect() stage to firebase serve to make sure functions and other emulators get set up the same as in firebase emulators:start
• Adds a devDependency on firebase-functions-test to the default template since it's required by the new functions emulator.

[coveralls]

Coverage increased (+0.02%) to 60.746% when pulling bab0baa on ss-emulators-serve-fixes into 81d8880 on master.

[coveralls]

Coverage decreased (-0.03%) to 60.635% when pulling 472d233 on ss-emulators-serve-fixes into fd11c78 on master.