agl-test-framework: add three build modes

agl-test-framework: add three build modes

we now add 3 build time modes for users to select:

mode 1: fast
register AGL_TEST_MODE = "fast" into the local.conf,
simply install tests runtime from 0-30 seconds.

mode 2: standard ---> DEFAULT
install tests that gets a runtime from 0 secs to 30 mins.

mode 3: extreme
install all tests.

---Notice---:
we set default to "standard" mode.
If you wish for other modes, put the below comment in your
conf/local.conf:
AGL_TEST_MODE = "fast" or AGL_TEST_MODE = "extreme"

Bug-AGL: SPEC-4753

Signed-off-by: Yan <yanxk.fnst@fujitsu.com>
Change-Id: If11db7ec50171aa69b98e1e656043722e2900182