MsgTriggerLaunch and MsgRevertLaunch tests: allow custom block timestamp and height for testing

[lumtis]

MsgTriggerLaunch and MsgRevertLaunch contain logic that depends on the context of the block (height and timestamp) (height since #581)

We currently set constant value for the context for testing in keeper test package:

var (
	// ExampleTimestamp is a timestamp used as the current time for the context of the keepers returned from the package
	ExampleTimestamp = time.Date(2020, time.January, 1, 12, 0, 0, 0, time.UTC)

	// ExampleHeight is a block height used as the current block height for the context of test keeper
	ExampleHeight = int64(1111)
)

For more reliable test we should allow to customize this value in the test case of the Table Tests, like for MsgTriggerLaunch for example:

for _, tc := range []struct {
  name string
  msg  types.MsgTriggerLaunch
  blockHeight int64
  blockTimestamp int64
  err  error
}{

With the current design of our tests, this change is actually not trivial and would require to work on #556