lit: add page (#15381)

Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>
Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

pages/common/lit.md

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+# lit
+
+> LLVM integrated tester for executing LLVM and Clang style test suites, summarizing results.
+> Part of LLVM.
+> More information: <https://www.llvm.org/docs/CommandGuide/lit.html>.
+
+- Run a specified test case:
+
+`lit {{path/to/test_file.test}}`
+
+- Run all test cases in a specified directory:
+
+`lit {{path/to/test_suite}}`
+
+- Run all test cases and check the wall time for each cases, then report to summary output:
+
+`lit {{path/to/test_suite}} --time-tests`
+
+- Run individual tests with Valgrind (memory check and memory leak test):
+
+`lit {{path/to/test_file.test}} --vg --vg-leak --vg-args={{args_to_valgrind}}`