[SYCL][PI] Fix PI unittests after PI API name changes (#572)

Adapt namechanges, e.g., `cl::sycl::detail::pi::piInitialize` to`cl::sycl::detail::pi::initialize`.

Fix unittests signed-unsigned comparison compiler warning in PI unittests:

```sh
warning: comparison between signed and unsigned integer expressions
 [-Wsign-compare]
```

Signed-off-by: Bjoern Knafla <[email protected]>

Author: bjoernknafla

sycl/unittests/pi/PlatformTest.cpp

@@ -20,7 +20,7 @@ class PlatformTest : public ::testing::Test {
 constexpr static size_t out_string_size =
     8192u; // Using values from OpenCL CTS clGetPlatforms test
 
-  PlatformTest() { detail::pi::piInitialize(); }
+  PlatformTest() { detail::pi::initialize(); }
 
   ~PlatformTest() = default;
 };
@@ -32,7 +32,7 @@ TEST_F(PlatformTest, piPlatformsGet) {
             PI_SUCCESS)
       << "piPlatformsGet failed";
 
-  ASSERT_GT(platformCount, 0) << "piPlatformsGet found 0 platforms.\n";
+  ASSERT_GT(platformCount, 0u) << "piPlatformsGet found 0 platforms.\n";
 
   std::vector<pi_platform> platforms(platformCount);