[docs] Fix TestSuiteGuide.md to mention scipy

[russell.gallop]

Summary

https://llvm.org/docs/TestSuiteGuide.html describes how to run compare.py in llvm-test-suite:

# Make sure pandas is installed. Prepend `sudo` if necessary.
% pip install pandas
# Show a single result file:
% test-suite/utils/compare.py results.json
# Compare two result files:
% test-suite/utils/compare.py results_a.json results_b.json

Since https://reviews.llvm.org/D57828 this also requires scipy so added this to the documentation.

[fhahn]

LGTM, thanks for updating the docs!

I guess it would be even better if we would provide the dependencies in a machine-readable way, e..g. https://python-packaging.readthedocs.io/en/latest/dependencies.html

Patches welcome :)

[russell.gallop]

In D82379#2153812, @fhahn wrote:

LGTM, thanks for updating the docs!

I guess it would be even better if we would provide the dependencies in a machine-readable way, e..g. https://python-packaging.readthedocs.io/en/latest/dependencies.html

Patches welcome :)

Thanks for the review. I agree. This was a drive by fix. I'll consider that if I spend more time in this area.

llvm/docs/TestSuiteGuide.md

5. Run the tests with lit:
    **********
    PASS: test-suite :: MultiSource/Applications/ALAC/encode/alacconvert-encode.test (2 of 474)
    ...
    ```
 
 6. Show and compare result files (optional):
 
    ```bash
-   # Make sure pandas is installed. Prepend `sudo` if necessary.
-   % pip install pandas
+   # Make sure pandas and scipy are installed. Prepend `sudo` if necessary.
+   % pip install pandas scipy
    # Show a single result file:
    % test-suite/utils/compare.py results.json
    # Compare two result files:
    % test-suite/utils/compare.py results_a.json results_b.json
    ```
 
 
 Structure