Rename $TEST_DIR to $SYSTEMD_TEST_DATA, document it

TEST_DIR is rather generic, and we prefix all variables used by installed
executables with "SYSTEMD_".

ENVIRONMENT.md

@@ -59,3 +59,8 @@ systemd-logind:
 * `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
   hibernation is available even if the swap devices do not provide enough room
   for it.
+
+installed systemd tests:
+
+* `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
+  a test executable is moved to an arbitrary location.

Makefile.am

@@ -152,7 +152,7 @@ endif
 AM_TESTS_ENVIRONMENT = \
 	export SYSTEMD_KBD_MODEL_MAP=$(abs_top_srcdir)/src/locale/kbd-model-map; \
 	export SYSTEMD_LANGUAGE_FALLBACK_MAP=$(abs_top_srcdir)/src/locale/language-fallback-map; \
-	export TEST_DIR=$(abs_top_srcdir)/test; \
+	export SYSTEMD_TEST_DATA=$(abs_top_srcdir)/test; \
 	export PATH=$(abs_top_builddir):$$PATH;
 
 if ENABLE_BASH_COMPLETION

src/shared/tests.c

@@ -43,7 +43,7 @@ const char* get_exe_relative_testdata_dir(void) {
         assert_se(readlink_and_make_absolute("/proc/self/exe", &exedir) >= 0);
         assert_se(snprintf(testdir, sizeof(testdir), "%s/testdata", dirname(exedir)) > 0);
         if (access(testdir, F_OK) < 0) {
-                fprintf(stderr,  "Test data directory '%s' does not exist, set $TEST_DIR\n", testdir);
+                fprintf(stderr,  "Test data directory '%s' does not exist, set $SYSTEMD_TEST_DATA\n", testdir);
                 exit(1);
         }
         return testdir;

src/test/test-helper.h

@@ -43,4 +43,4 @@
                )
 
 #define TEST_DATA_DIR(subdir)                                     \
-        strjoina(getenv("TEST_DIR") ?: get_exe_relative_testdata_dir(), subdir)
+        strjoina(getenv("SYSTEMD_TEST_DATA") ?: get_exe_relative_testdata_dir(), subdir)