tests/cmdline: Add test for MICROPYINSPECT environment variable.

When this variable is set to non-empty string it triggers the REPL after a
command/module/file finishes running.

The Python file without the file extension is because the cmdline: parser
in run-test splits on spaces, so we can't use the -c option since
`import os` can't be written without a space.

tests/cmdline/repl_micropyinspect

@@ -0,0 +1,3 @@
+import uos
+
+uos.putenv('MICROPYINSPECT', '1')

tests/cmdline/repl_micropyinspect.py

@@ -0,0 +1,2 @@
+# cmdline: cmdline/repl_micropyinspect
+# setting MICROPYINSPECT environment variable before program exit triggers REPL

tests/cmdline/repl_micropyinspect.py.exp

@@ -0,0 +1,5 @@
+MicroPython \.\+ version
+Use \.\+
+>>> # cmdline: cmdline/repl_micropyinspect
+>>> # setting MICROPYINSPECT environment variable before program exit triggers REPL
+>>>