Call ob_start() before running tests

Call ob_start() before running tests

The policy introduced for T206476 creates a subtle failure mode: any test
writing to stdout will cause headers to be sent, causing later tests to
fail when they try to call header().

Instead, call ob_start() to intercept test output. Any buffered output is
still seen when PHPUnit exits.

Bug: T206476
Change-Id: Id085efeab67d1e700ffcbf37868b5107e3a7e5d5