tests: move to subdir, move to meson

De-clutter the source directories a bit. Ensure tests build with meson, and
remove from autotools in a few places (no need to do things twice).

mu/Makefile.am

@@ -68,38 +68,5 @@ EXTRA_DIST=                                                     \
 	mu-help-strings.awk                                     \
 	mu-help-strings.txt
 
-#
-# Tests
-#
-
-noinst_PROGRAMS= $(TEST_PROGS)
-
-test_cxxflags=                                                  \
-	${AM_CXXFLAGS}                                          \
-	-DMU_TESTMAILDIR=\"${abs_top_srcdir}/lib/testdir\"      \
-	-DMU_TESTMAILDIR2=\"${abs_top_srcdir}/lib/testdir2\"    \
-	-DMU_TESTMAILDIR4=\"${abs_top_srcdir}/lib/testdir4\"    \
-	-DMU_PROGRAM=\"${abs_top_builddir}/mu/mu\"              \
-	-DABS_CURDIR=\"${abs_builddir}\"                        \
-	-DABS_SRCDIR=\"${abs_srcdir}\"
-
-TEST_PROGS += test-query
-test_query_SOURCES= tests/test-mu-query.cc
-test_query_CXXFLAGS=$(test_cxxflags)
-test_query_LDADD=${top_builddir}/lib/libtestmucommon.la $(CODE_COVERAGE_LIBS)
-
-TEST_PROGS += test-cmd
-test_cmd_SOURCES= tests/test-mu-cmd.cc
-test_cmd_CXXFLAGS=$(test_cxxflags)
-test_cmd_LDADD=${top_builddir}/lib/libtestmucommon.la $(CODE_COVERAGE_LIBS)
-
-TEST_PROGS += test-cmd-cfind
-test_cmd_cfind_SOURCES= tests/test-mu-cmd-cfind.cc
-test_cmd_cfind_CXXFLAGS=$(test_cxxflags)
-test_cmd_cfind_LDADD=${top_builddir}/lib/libtestmucommon.la $(CODE_COVERAGE_LIBS)
-
-TESTS=$(TEST_PROGS)
-include $(top_srcdir)/aminclude_static.am
-
 CLEANFILES=                                                     \
 	$(BUILT_SOURCES)