[2.74] test-utils: Shut down Apache server more reliably

• tests: add soup_test_build_filename_abs()

  And use it when an absolute patch is required.

  (cherry picked from commit 235160bc)

  From: @carlosgc

• test-utils: Log Apache arguments

  This helped to diagnose a use-after-free bug in the unit tests.

• test-utils: Save Apache server root during initialization

  By the time we want to use it during teardown, it is no longer safe to call g_test_build_filename(), because some GTest state is discarded at the end of g_test_run() (see glib#2563 (closed)). As a result, the command that should shut down the server fails, the server continues to run, and its port is unavailable for the next test-case.

  This should mostly resolve #175.

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Backport of !265 (merged) to 2.74.x (which is the only branch we have in Debian right now).