Reduce the number of frames tested in EncoderInterfaceTest

There's little point in running the same test over and over for
a huge number of frames if it doesn't test much different things.

This reduces the runtime of EncoderInterfaceTest.* from 322 seconds
to 140 seconds, when running in valgrind.
Set a specific frame size in TemporalLayerSettingTest

Previously, this test used whatever size was set in m_iWidth
and m_iHeight before, which depended on the order that the tests
were executed. When this test was the first one executed in the
EncoderInterfaceTest, the width and height were set to the max.

Instead of having the test behaviour depend on the test order,
set a specific size, just as InitializeParamExt and MemoryCheckTest
do.

This reduces the runtime of TemporalLayerSettingTest from 86 seconds
to 26 seconds, when run in valgrind.
Merge pull request #1256 from mstorsjo/speedup-tests

Speed up test cases in EncoderInterfaceTest