Tests: Add base structure for LibPDF unit tests

Add a unit test for each sample pdf file that currently exists in the
anon user's `~/Document/pdf` directory.
- linear.pdf
- non-linearized.pdf
- complex.pdf

Each test ensures that the pdf document is parsed and that the page
count is the expected one.

Tests/LibPDF/CMakeLists.txt

@@ -0,0 +1,13 @@
+set(TEST_SOURCES
+    TestPDF.cpp
+)
+
+foreach(source IN LISTS TEST_SOURCES)
+    serenity_test("${source}" LibPDF LIBS LibCore LibPDF)
+endforeach()
+
+install(
+    FILES "../../Base/home/anon/Documents/pdf/complex.pdf"
+          "../../Base/home/anon/Documents/pdf/linearized.pdf"
+          "../../Base/home/anon/Documents/pdf/non-linearized.pdf"
+    DESTINATION usr/Tests/LibPDF)