allow seting custom API timeouts

[leoleovich]

Summary: For config push, firmware upgrade and metrics export we have to have large timeout values (4 minutes). But for the rest we don't. In real world it results in tests being slow running up to 4 minutes per test:

# go test github.com/facebook/time/calnex/verify/checks -v
=== RUN   TestGNSS
--- PASS: TestGNSS (75.00s)
=== RUN   TestHTTP
--- PASS: TestHTTP (75.01s)
=== RUN   TestPing
--- PASS: TestPing (1.01s)
=== RUN   TestPSU
--- PASS: TestPSU (75.00s)
PASS
ok      github.com/facebook/time/calnex/verify/checks   226.514s

Lowering most timeouts expect for aforementioned.

Reviewed By: deathowl

Differential Revision: D52661410

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D52661410

[facebook-github-bot]

This pull request has been merged in facebook/time@164991709928e578a846fad8acc8079ed466c401.