Add simple test for the dive planner

This sets up a standard dive scenario (30 minutes at 260ft/79m, EAN36 and
Oxygen as deco gases, last stop at 20ft/6m) and calls the planner to set
up a dive plan given certain standard gases.

Instead of trying to verify the complete plans it checks that we switch to
the deco gases at the right depth and the complete duration of the dive
matches our expectation.

The test intentionally fails right now for imperial as we have the wrong
switch depth for Oxygen. See how useful tests are?

On the downside, the test does NOT produce the same plan as Subsurface
when I try to create a consistent setup for both - and I have not been
able to figure out why. There must be some other parameters that I'm not
setting, but I haven't identified them, yet. It's very small differences,
for example in the metric case the stops at 21m, 9m, and 6m are each one
minute shorter in the test than it what Subsurface calculates.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

tests/testplan.h

@@ -0,0 +1,13 @@
+#ifndef TESTPLAN_H
+#define TESTPLAN_H
+
+#include <QTest>
+
+class TestPlan : public QObject {
+	Q_OBJECT
+private slots:
+	void testImperial();
+	void testMetric();
+};
+
+#endif // TESTPLAN_H