test_e2e.sh: quote variables to prevent globbing and word splitting

[rul]

Fixes SC2086. Part of #891.

git-sync#fix-e2e-sc2086> shellcheck -i SC2086 test_e2e.sh  && echo $?
0

As explained in 390f63c, some strings don't require to be fully quoted, but doing
so anyway to keep code consistency.

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[thockin]

Thanks!

/lgtm
/approve

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