selftests: kselftest framework: add handling for TAP header level

Introduce environment variable KSFT_TAP_LEVEL to avoid printing
nested TAP headers for each test. lib.mk run_tests target prints
TAP header before invoking the test program or test script. Tests
need a way to suppress TAP headers if it is already printed out.

This new environment variable adds a way for ksft_print_header()
print TAP header only when KSFT_TAP_LEVEL isn't set.

lib.mk run_tests and test program should print TAP header and set
KSFT_TAP_LEVEL to avoid a second TAP header to be printed.

selftests Makefile should export KSFT_TAP_LEVEL and add TAP Header
echo to the run_kselftest.sh script from emit_tests target handling.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/kselftest.h

@@ -57,6 +57,7 @@ static inline int ksft_get_error_cnt(void) { return ksft_cnt.ksft_error; }
 
 static inline void ksft_print_header(void)
 {
+	if (!(getenv("KSFT_TAP_LEVEL")))
 		printf("TAP version 13\n");
 }