Adding unit test for mbedtls_x509write_csr_set_extension()

The already existing "x509_csr_check()" function is extended in order
to support/test also CSR's extensions. The test is performed by
adding an extended key usage.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>
diff --git a/tests/data_files/server1.req.sha256.ext b/tests/data_files/server1.req.sha256.ext
new file mode 100644
index 0000000..3f26f09
--- /dev/null
+++ b/tests/data_files/server1.req.sha256.ext
@@ -0,0 +1,17 @@
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+-----END CERTIFICATE REQUEST-----