KVM: selftests: Generalize check_clocksource() from kvm_clock_test

Several existing x86 selftests need to check that the underlying system
clocksource is TSC or based on TSC but every test implements its own
check. As a first step towards unification, extract check_clocksource()
from kvm_clock_test and split it into two functions: arch-neutral
'sys_get_cur_clocksource()' and x86-specific 'sys_clocksource_is_tsc()'.
Fix a couple of pre-existing issues in kvm_clock_test: memory leakage in
check_clocksource() and using TEST_ASSERT() instead of TEST_REQUIRE().
The change also makes the test fail when system clocksource can't be read
from sysfs.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240109141121.1619463-2-vkuznets@redhat.com
[sean: eliminate if-elif pattern just to set a bool true]
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/lib/test_util.c

@@ -392,3 +392,28 @@ char *strdup_printf(const char *fmt, ...)
 
 	return str;
 }
+
+#define CLOCKSOURCE_PATH "/sys/devices/system/clocksource/clocksource0/current_clocksource"
+
+char *sys_get_cur_clocksource(void)
+{
+	char *clk_name;
+	struct stat st;
+	FILE *fp;
+
+	fp = fopen(CLOCKSOURCE_PATH, "r");
+	TEST_ASSERT(fp, "failed to open clocksource file, errno: %d", errno);
+
+	TEST_ASSERT(!fstat(fileno(fp), &st), "failed to stat clocksource file, errno: %d",
+		    errno);
+
+	clk_name = malloc(st.st_size);
+	TEST_ASSERT(clk_name, "failed to allocate buffer to read file");
+
+	TEST_ASSERT(fgets(clk_name, st.st_size, fp), "failed to read clocksource file: %d",
+		    ferror(fp));
+
+	fclose(fp);
+
+	return clk_name;
+}