[dotest] Make --test-subdir work with --no-multiprocess

The single-process test runner is invoked in a number of different
scenarios, including when multiple test dirs are specified or (afaict)
when lit is used to drive the test suite.

Unfortunately the --test-subdir option did not work with the single
process test runner, breaking an important use case (using lit to run
swift-lldb Linux tests):

  Failure URL: https://ci.swift.org/job/swift-PR-Linux/6841

We won't be able to run lldb tests within swift PR testing without
filtering down the set of tests.

This change makes --test-subdir work with the single-process runner.

llvm-svn: 339929