tests: do not reuse the same directory between two tests

By accident, both health_output-led and sysrepo_system-leds tried to put
their temporary sysfs-ish sample files into a subdirectory with the same
name. That is prone to races, and races I saw (unrelated output removed):

       Start 22: sysrepo:prep:sysrepo_system-leds
 11/58 Test #22: sysrepo:prep:sysrepo_system-leds ...........................   Passed    0.20 sec
       Start 21: test-sysrepo_system-leds
       Start  4: test-health_output-led
 32/58 Test  #4: test-health_output-led .....................................   Passed    0.05 sec
 36/58 Test #21: test-sysrepo_system-leds ...................................***Failed    0.28 sec
 50.970 [749106 system W] Failed reading state of the LED 'line:red': File '/home/jkt/work/prog/_build/czechlight-asan-ubsan-ly2/velia/tests/leds/line:red/brightness' does not exist.

I pondered using __FILE__ for this, but since __FILE_NAME__ is only in
clang and I don't feel like spending too much time here, a simple
hardcoded thingy will have to suffice.

Change-Id: Ib0cde221a844a662aa58f8dee88500d34b533201
diff --git a/tests/health_output-led.cpp b/tests/health_output-led.cpp
index 3139be7..63eb0d8 100644
--- a/tests/health_output-led.cpp
+++ b/tests/health_output-led.cpp
@@ -39,7 +39,7 @@
     using namespace std::literals;
     TEST_INIT_LOGS;
 
-    auto fakeSysfsDir = std::filesystem::path {CMAKE_CURRENT_BINARY_DIR + "/tests/leds/"s};
+    auto fakeSysfsDir = std::filesystem::path {CMAKE_CURRENT_BINARY_DIR + "/tests/health_output-led/"s};
     auto fakeBrightnessFile = fakeSysfsDir / "brightness";
     removeDirectoryTreeIfExists(fakeSysfsDir);