test-reach: add run_three_modes method

test-reach: add run_three_modes method

The 'test_three_modes' method assumes we are using the 'test-tool
reach' command for our test. However, we may want to use the data
shape of our commit graph and the three modes (no commit-graph,
full commit-graph, partial commit-graph) for other git commands.

Split test_three_modes to be a simple translation on a more general
run_three_modes method that executes the given command and tests
the actual output to the expected output.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>