Streamline Junit Tests - StyledText (part 1)

StyledText contains many assertTrue statements that need to be changed
over to assertEquals. This patch contains the first set of changes.

Change-Id: Ifcebc469b29c2b69afd4d324fc3dd556f187a54f
Signed-off-by: jmajano <[email protected]>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.swt/+/186788
Tested-by: Platform Bot <[email protected]>
Reviewed-by: Alexander Kurtakov <[email protected]>

Author: joel-majano