[CredentialsTest] Remove captureDisplay

Replace ScreenCapture#captureDisplay with captureLayers.
CredentialsTest#CaptureLayersTest already uses
captureLayers, which makes CaptureTest unncessary. Test
with the full path because of multiple files with the same
test name.

Bug: b/293445881
Test: atest services/surfaceflinger/tests/Credentials_test.cpp
Change-Id: Ief4db33ab2a437cd8aec0ee35bac60ae966a4b94