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Some external module dependencies, notably numpy and pandas, are
required by some command line perf tools to work properly. And these
are normally satisfied by the vpython environment.

However vpython is not always available, e.g. on ChromeOs, so we
provide a mechanism to safely dismiss the respective import errors
and skip tests that cannot be run due to the lack of these modules.

Doing in this in preparation for the migration of soundwave out
of catapult/experimental and into tools/perf; since that includes
many other tests that also depend on these external modules.

TBR=achuith@chromium.org

Bug: 922030
Bug: 921762
Change-Id: I162e8e232484a5f9c8f919426ad86c6d0ad2e428
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1494887Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637648}