test: add test files for number_abs, string_contains, string_count and fix empty string corner case in string_contains primitive

test/number_abs.wren

@@ -0,0 +1,6 @@
+io.write(123.abs)    // expect: 123
+io.write(-123.abs)   // expect: 123
+io.write(0.abs)      // expect: 0
+io.write(-0.abs)     // expect: 0
+
+// TODO(bob): Floating point numbers.

test/string_contains.wren

@@ -0,0 +1,8 @@
+io.write("".contains(""))   // expect: 1
+io.write("anything".contains("")) // expect: 1
+io.write("something".contains("meth")) // expect: 1
+io.write("something".contains("some")) // expect: 1
+io.write("something".contains("ing")) // expect: 1
+io.write("something".contains("math")) // expect: 0
+
+// TODO(bob): Passing non-string as argument.

test/string_count.wren

@@ -0,0 +1,2 @@
+io.write("".count)   // expect: 0
+io.write("a string".count) // expect: 8