test: sysfs: Fix readability-identifier-naming

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../test/sysfs.cpp:29:25: error: invalid case style for variable 'MAX_BRIGHTNESS_VAL' [readability-identifier-naming,-warnings-as-errors]
constexpr unsigned long MAX_BRIGHTNESS_VAL = 128;
                        ^~~~~~~~~~~~~~~~~~
                        maxBrightnessVal
```

Change-Id: I8383e1476f31ff2c488b18a97d4a32294327a9c2
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/test/sysfs.cpp b/test/sysfs.cpp
index 5d83d80..75fbaf1 100644
--- a/test/sysfs.cpp
+++ b/test/sysfs.cpp
@@ -26,7 +26,7 @@
 
 namespace fs = std::filesystem;
 
-constexpr unsigned long MAX_BRIGHTNESS_VAL = 128;
+constexpr unsigned long maxBrightnessVal = 128;
 
 class FakeSysfsLed : public phosphor::led::SysfsLed
 {
@@ -64,7 +64,7 @@
         fs::path p = root / attrMaxBrightness;
         std::ofstream f(p, std::ios::out);
         f.exceptions(f.failbit);
-        f << MAX_BRIGHTNESS_VAL;
+        f << maxBrightnessVal;
     }
 };
 
@@ -81,7 +81,7 @@
 {
     FakeSysfsLed fsl = FakeSysfsLed::create();
 
-    ASSERT_EQ(MAX_BRIGHTNESS_VAL, fsl.getMaxBrightness());
+    ASSERT_EQ(maxBrightnessVal, fsl.getMaxBrightness());
 }
 
 TEST(Sysfs, getTrigger)