doc: simplify install instructions

[coryan]

When installing the library, do not run the unit tests or benchmarks.
This removes two steps from the installation instructions, even the more
difficult platforms have about 10 steps, so 2 steps is a significant
change.

This test theoretically reduces the testing coverage, as it disables unit
tests in a number of platforms. We think the risk is low, as the same unit
tests pass in many other builds, and we are running at least one smoke
test for the ci/kokoro/install builds.

This fixes #1216

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This change is 

[codecov-io]

Codecov Report

Merging #1268 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #1268 +/- ## ========================================== - Coverage 94.85% 94.84% -0.01%  ========================================== Files 181 181 Lines 13794 13798 +4 ========================================== + Hits 13084 13087 +3  - Misses 710 711 +1

Impacted Files	Coverage Δ
google/cloud/spanner/internal/spanner_stub.cc	69.56% <0%> (-1.09%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.55% <0%> (-0.16%)	⬇️
google/cloud/spanner/internal/polling_loop.h	91.89% <0%> (ø)	⬆️
google/cloud/spanner/database_admin_client.cc	100% <0%> (ø)	⬆️
google/cloud/spanner/session_pool_options.h	100% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.37% <0%> (ø)	⬆️
...anner/integration_tests/client_integration_test.cc	98.49% <0%> (+0.01%)	⬆️
...nner/benchmarks/single_row_throughput_benchmark.cc	91.82% <0%> (+0.02%)	⬆️

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